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LESSONS

i.\

ELEMENTARY PHYSIOLOGY

]:y

THOMAS II. HUXLEY, LL.D., F.R.S.

MACMILLAN AND CO., Limited

NEW YORK : THE MACMILLAN COMPANY
1898

All rishts 7-e served

Richard Ci.ay and Sons, Limited,
london' a^.■^ bl'ngay.

First Eilition printed xZtd. New Edition, 1868. Reprinted ii6g, 1870, 1871, March
and May, 1872. JA^ifTc Edition, October, 1872. Repritited, 1873, 1874, 1875, 1876,
1878, 1879. 1881, 1883, Jnniiary, Ecbruaiy, hfay, Septeiiiler^ and Xorz'eiul'cr, 1884.
Netv Edition, 1885. Ecprinied 1886, 1E88, 78:;o. 1892, January and September,

X893, 1S96, 1898.

PREFACE TO REVISED EDITION,
(1885).

It is now more than twenty years since I planned
and began to write these " Lessons," the object of
which is defined in the preface to the first edition.

During this period physiological investigations have
displayed an activity unprecedented in histon.'. Here,
as in all such branches of ratural knowledge, the
method of experiment has shown itself to be the one
path which leads to truth, and has not only revealed
multitudes of novel physiological facts, but has
suggested wholly new lines of inquiry.

As successive editions of the " Lessons " have
been demanded, it has been my effort to incorporate
with them such solid acquisitions of the ever advanc-
ing science of physiology as fall within their scope,
while rigorously excluding all debatable matter or, at
any rate, carefully indicating it as such. At the same
time, long experience as a Teacher and Examiner
having forcibly brought home to my mind the difficulty

y{ PREFACE.

of making any statement that cannot be misunder-
stood, an amount of attention has been devoted to
questions of mere exposition, which really deserves,
but probably has not attained, the reward of abolishing
such misunderstandings.

The present edition has been more extensively
revised than any of its predecessors. The chapter on
Histology, in particular, has been entirely reconstructed
and provided with new illustrations, several of which
have been taken from Ranvier and from Quain.

In the preface to former editions of this work, I
have had the pleasure of expressing my obligations
to Dr. Foster, Secretary to the Royal Society, for his
very valuable assistance. That aid lias been still
more freely rendered to the present edition, which,
in fact, could not have appeared unless Dr. Foster
had taken upon himself the whole burden of seeing
the work through the press. My friend has indeed
done so much during my enforced idleness, that I
should have been better satisfied if he would have
permitted me to associate his name with mine on
the title-page.

T. H. HUXLEY.

Rome, February^ 1SS5,

CONTEXTS.

LESSON J.

A General \'iew of the Structure and Functions
OF THE Human Body. Pp. i — 21.

§ I. Modis of studying the action of mans body.

2. Purpose of these Lessons.

3. Experimental proof t/iat a living active man gives

out heat, exerts mechanical force, and loses sub-
stance in the form of carbonic acid, water, and
other matters.
4, 5 . These losses made good by the taking in of air, drink,
and food.

6. Balatice of bodily iticome and expenditure.

7. Work and Waste ; the body compared to a steam-

engine.

8. Ge?ieral build of the body — head, trunk, and limbs.

9. The vertebrce and spinal cord. The cavities of the

trunk.

10. TJie human body a double tube.

1 1 . The tissues. Integument.

12. Connective tissue.

13. Muscle.

1 4. The skeleton.

15. The maintenance of an upright positio.i the result

of many combined actions.

16. The relation of the mind to the action of the

muscles.

viii CONTENTS.

§ 17. The spinal cord capable of convert i tig impressions
from without into muscular contractions.

1 8. special sensations.

1 9. The tissues are constantly being refiewed.

20. The renewal is effected by means of the alimentary

apparatus, tuhich cotiverts food into jiutriment j
ajid by the
21, 22. Organs of circulatio?i, whicJi distribute the jiutri-
ment o7'er the body.

23. The excretory organs drain luaste matters from the

body.

24. Double fujiction of the lungs.

25. The neri'ous system combines the actioji of the

various organs.

26. Life and death.

27. Local death constantly going on iti the body.

28. General death — deat/i of the body as a luhole, and

death of the tissues.

29. Modes of death.

30. Decomposition and diffusion.

LESSON IT.

The Vascular System and the Circulation.

Pp. 22—59.

§ I. The nature and arrangement of the capillaries.

2. Structure and properties of arteries and veins.

3. Differences betiveen arteries and veins.

\. Structure and function of the valves of the veins.

5. The Lymphatics.

6. The Lactcals.

7. A general vieiu of the way in which the vessels are

arranged in the body and are cotinected with the
heart.
8, 9. The Heart, its connexions and structure ; f he peri-
cardium a?id endocardium; the auricles and
ventricles.

10. Its valves, their structure, action, and purpose.

1 1 . Its systole atid diastole.

CONTENTS. IX

§ 12. The iuo7-king of the heart J the inecJumisni bywhuh
the hearty through its contractions^ drives the blood
always i?i one direction^ explained.

13. The worki)ig of the arteries.

14. The beat of the heart.

1 5. The sounds of the heart.

1 6. The pulse in the arteries.

17. Why blood flows in jej'ks from a cut artery.

18, 19. Why no pulse is present i?i the capillaries and veins.
20. The rate at which the blood flows in the diferent
blood-vessels.
21, 22. The circulation traced in its whole course.

23. Tlie nervous system regulates the calibre of the

small arteries, afid thereby controls the flow of
blood through various parts : blusht?ig, &^c.

24. Experimental proof of this.

25. The results of this controlling power of vaso-motor

nc7'ves.
26, 27. The 7?tove?nents of the heart are also under the
co7itrol of the nervous system,
28. The proofs of the circulation. Direct observation
of the circulation of the blood iti the web of a
frog^sfoot.

LESSON III.

The Blood and the Lymph. Pp. 60—76.

§1 — 3. The prope7'ties of a drop of blood: co7'puscles,
plas}7ia, coagulatio7i.
4. Red corpuscles.
5, 6. Colourless corpuscles ; their contractility.

7. Developme7it of cot'Puscles j the red corpuscles a7'e

probably derived fro})i tJie colourless 07ies.

8. Red co7puscles of shed blood te7id to stick together in

rolls.

9. Blood-crystals, licenuiglobin.

10, II. Coagulatio7i of blood jfibri7i, crassame7ituf7i or clot,
seru/fi.
12. Buf/y coat.

CONTENTS.

§ 13. Itijluoice of circumstafices on the j'apidity of
coagulation.

14. Nature of the process of coagulation; globulin,

fibrinogen.

15. The physical qualities of the blood.

1 6. The chemical cotnpositio?! of the blood.

17. Infiuence of age, sex, food, dr^c. on the blood.
I S. Total quantity of blood i?t the body.

19. The vii'ifying influence of blood over the tissues :

transfusion.

20. The Lymph.

LESSON IV.

Respiration. Pp. 77—105.

§ I. The blood a highly complex product derived from
all parts of the body.

2. Blood rendered 7 mo us in the capillaries.

3. Difference between arterial a?id vettous blood.

4. Nature of the change of venous blood into arterial

and vice versa.

5. Cause of change in colour of blood.

6. Blood is changed from arterial to venous in the

systejnic, and from venous to arterial in the pul-
mofiary capillaries.

7. The essence of respiration.

8. Machifieiy of respiration. The air-passages and

chambers.

9. Necessity for the renewal of the air in the lungs.

10. The respiratory act ; inspiration, expiration.

1 1. Differences between inspired and expired air.

1 2. 7 he amount of work done by the lungs.

1 3. The mechanism of the respiratory mo^'ements. The

elasticity of the lungs.
14- Contractility of the walls of the bronchial tubes.
Ciliary action.

15. Moz'ements of the chest-walls. The intercostal

muscles.

16. The diaphragm.

CONTENTS. xi

§ 1 7. Action of the diaphrag})i and intercostal fnuscies
compared.

18. Accessory muscles of respiratio7i.

19. SigJmig, coughing, <r^c.

20. The chest compared to a bellows. Residual, supple-

mental, complemental, tidal, and stationary air.

21. The statio?iary air plays the part of a middle

man.

22. Composition of stationary air.

23. The respi?-atory mechanism under the control of the

neri'ous system.
24, 25. Respiration and circulation compared.

26. The respiratory murmurs.

27. hispiratiofi assists the circulation.

28. Effect of expiration on the circulation.

29. The activity of the respiratory process moaijied by

the circumstances of life.
30,31. Asphyxia.

32. The two influences, deprivation of oxygen and
accumulation of carbonic acid.
Ii)iporta?ice of the foriner.

jj-

34. Necessity for an abimdance of fresh air.

LESSON V.

The Sources of Loss and of Gain T'j the Blood.
Pp. 106—142.

§ I. Distribution of arterial blood.
2—4. TJie blood in various ways meets with constant
or interynittent gains and losses of material and
heat.

5. Tabular view of the sources of loss and gain.

6. The loss by the kidneys. The urinary apparatus.

7. Composition of urine.

8. Kidneys and lungs co;npared.

9. The structure of the kidney.

10 12. Nature of the act of secretion by the kidney.

1 3. The loss by the ski?!. Sensible and i?isensible per-

spiration.

14. Quantity and composition of sweat.

xii CONTENTS.

§ 13. Perspiration by simple transudation.

16. Sweat-o^lafids.

17. These glands are controlled by the nervous system.

18. Variations i7i the quantity of matter lost by per-

spiratioji.

19. The lu/igs, skin, and kidneys compared together.

20. The liver, its connexions and structure.

2 1 . The active power of the live?'-cells.

22. The bile. Its quantity ajid composition.

23. Bile is formed in the liver-cells.

24, 25. Other cha7iges in the blood effected by the hepatic
cells. Experi))iental proof of the formation of
sugar in the liver. Glycogen.

26. Sources of gai7i of matter. Gain of oxygen to the

blood through the lungs.

27. Gain by the lymphatics.

28. Tlie spleen.

29. Gain of heat. Generatioti of heat by oxidation.

30. Distributioti of Jieat by the blood current.

31. Temperature of the body regulated by meafis of the
nervous system.

The glands are intcr)nittently active sources of loss.

Structure and functions of glands, nature of act

of secretion.
Gain of ivaste products from the muscles.

LESSON VI.
The Fun'CTIon of Alimentation. Pp. 143— 1 68.

§ F . TJie alimentary canal, the chief source of gain.

2 . The quantity of dry, solid, afid gaseous aliment daily

taken i?i by a man.

3. The quantity of dry solid matter daily lost by a

man.

4. Classification of aliments. The chief 7'ital food-

stuffs: — Proteids, Fats, Amyloids, ^linerals.

5. Their ultimate analysis. The presence of Proteids

and Minerals in food indispensable.

6. Xo absolute necessity for other food-stuffs.

J—

CONTEXTS. xiii

§ 7. Nitrogen starvation.

8. Disadvantages of a purely 7iitrogenous diet.

9. Economy of a mixed diet.

10. Advatitage of combining different articles of food.

1 1 . Intermediate cha?iges undergone by food in the

body.

12. Division of foodstuffs into heat-producers and

tissue-formers misleading.

1 3. Function of the alimenta?y apparatus. The mouth

and pharynx.

1 4. The salivary glands.

15. The teeth.

1 6. Eati?ig a7id siv allowing.

17. Drinking.

18. The stomach and tlic gastric juice.

1 9. A rtificial digest i 071.^ pepto7ic.

20. Chy77ie. Absorptio7i from the st07)iach.
1 1 . The large a7id S7nall i7itestines.

22. The intestinal glands and juice. The valvule?

conniventes and villi. Pe7-istaltic contraction.

23. E7it ranee of bile ajid pancreatic juice.

24. Actio7i of these fluids. The villi. Absorptio7ifro7n

the intestines.

25. Digestio7i i7i the large intestine.

LESSON VII.

Motion and Locomotion. Pp. 169 — 200.

§ I . The vital eddy. The source of the active powers of
the economy.

2. The orga7is of 77iotio7i are cilia and 7nuscles.

3. Cilia.

4. Muscles. Muscular contractt07i. Rigor 7nortis,

5. Hollow 77iuscles.

6. Muscles attached to levers. The three orders of

levers.

7. Examples J in the body^ of levers of the first order.

8. Exa77iples of levers of the second order.

9. Exa77iples of levers of the third orde7\

xiv CONTENTS.

§ lo. The same parts may i-epresc7ity in tur?i, each of the
three orders.

1 1. Joints or articulations. Imperfect joints.

12. Structure of perfect joints.

1 3. Ball and socket joifits.

14. Hinge joints.

1 5. Pivot joiiits. The atlas and axis. The radius a?id

ulna pronation attd supi7iatio?i.

16. Ligaments.

17. Various ki7ids of inovements of joints.

18. Means of effect i7ig the/n.

1 9. Tendons.

20. lValki7tg^ run?ii7ig, ju77ipi7tg.

21. Co7iditio7is of the product io7i of the Voice.

22. The vocal chords.

23. The cartilages of the larynx.

24. The 77mscles of the lafynx. The action of the

several parts of the larynx.

25. High a7id low 7iotes ; ra7ige a7id quality of voice.
26, 27. Speech. Production of vowel soimds a7id co7i-

ti7lU0US C07lS07ia7ltS.

28. Explosive co7iso7iants.

29. Speaki7ig 7nachi7ies.

30. To7igueless speech.

LESSON VIII.

Sensations and Sensory Organs. Pp. 201—240.

§1,2. Ani7nal movements the result of a series of cha7tges
usually origi7tated by exter7ial imp7-cssio7is.

3. Reflex actio7i. Se7isations and co7isciousness.

4. Subjective sensatio7is.

5. The 77iuscular seiise.

6. The higher se7ises.

7. Ge7teral plan of a se7tsory 07gan : essential and

accessory parts.

8. Touch. PapHlce. Tactile corpuscles, and end

bulbs.

9. Functio7i of the epitheliu7>i.

10. Touch 77iore acute i/t some parts of the skin than in
others.

CONTENTS. XV

§ 1 1. TJie sense o/iL>a?-?nth or cold.

12. Taste. The PapUlcE of the tongue^ iastebuds.

13. Smell. The anatomy of the nostrils. The iur-

binal bones. The olfactoty and non-olfactory
7nucous 7nembranc.

14. The reason of^^ sniffing:'

1 5. The essential parts of the organ ^Hearing ; the

auditory epithelium, perilymph, endolymph. What
takes place in hearing.

1 6. The vestibule and sonicircular canals. The mem-

branous and osseous labyrinth. The endings of
the auditory nerve in the cristae and maculae
acustica?.

17. The cochlea. The scala tympani^ scala vestibuli,

scala media.
I S. The orga7i of Corti.

19. The fenestra rotunda and fenestra 07'alis.

20. The external meatus, tympa?ium, and Eustachian

tube.

21. The auditory ossicles.

22. The muscles of the tympanum.

23. The concha.

24. Nature of sound. Vibrations of the tyfftpanic

membrane.
25-27. Transmission of the vibratiofis of the tympanum.

The action of the auditory ossicles.
28. How vibratio7is of a sounding body give rise to

sensations of soujid.
29-30. Respective functiojis of 77ie77ibranous labyri7ith and

cochlea.

3 1 . Subjective audit 07y se7isatio7is.

32. The functio7tsofthe ty77ipanic 7nuscles. The Eusta-

chian tube.

LESSON IX.
The Organ of Sight. Pp. 241—264.

§ I. Ge7te7'al structure of the eye.

2. The surface of the reti7iaj the i7tacula lute a,

3. Microscopic structure of the 7rtina.

4. The sensatio7i of light.

xvi CONTENTS.

§ 5. The '■' blind spot.'-

6. Duration of a luminous impression.

7. Exhaustion of l/ie retina. Co7nplementary colours.

8 . Colour blindness.

9. Sensations of light from pressure on the eye.

Ph'osphenes.
10. Functions of the rods and cones. The figures oj
PurJdnje.
1 1 - 1 3. TJie p)ropcrtics of lenses.

14. The i filer mediate apparatus. The eyeball. The

sclerotic and cornea.

1 5. 7 he aqueous and 7'itreous humours. The crystal-

line lens.

1 6. The choroid and ciliary processes.

1 7. The iris and ciliary muscle.

1 8. The iris a self-regulating diaphragm.

1 9. Focal adjustment.

20. Experiment illustrating the power of adjustment

possessed by the eye.

21. 7 'he jnechajiism of adjustment explained.

22. Limits of the power of adjustment. Long and short

sight.

23. The muscles of the eyeball; their action.

24. The eyelids.

25. The lachrymal apparatus.

LESSON X.
The Coalescence of Sensations with one another

AND with other STATES OF CONSCIOUSNESS. Pp.
265—277.

§ I. Many apparently simple sensations arc, in reality,
composite.

2. The sensations of smell the least complicated.

3. Analysis of the sensation obtained by draiving the

finger along a table.

4. The notion of rouiidness a 7'ery complex judgment ;

Aristotle's experime?it.

5. '"'' DelusioJis of the senses'' i?i reality delusiofis of

the judgment.

CONTENTS. xvii

§ 6. Sicbjcctive sensations ; delusions of f/ie' jud^niefit
through abnornuil bodily conditio7is. Audilory
and ocular spectra.

7. Case of Mrs. A. related by Sir David Brewster.

8. Ventriloquism.

9. Optical delusions.

10. Visual images referred to some point without the

body.
I r , The inversio?i of the visual image.

1 2. Distinct visual images referred by the mind to dis-

tinct objects. iMultiplying glasses.

1 3. The judgjnent of distance by the size and intensity

of visual images. Perspective.

14. Effects of convex atid concave glasses.

15 Why the sun, or moon., looks large near the
horizon.

16. The judgment of form by shadows.

1 7. The judgmc7it of changes of form. TJie thau-

m at rope.

1 8. Si?igle vision with two eyes. Corresponding points.

1 9. The pseudoscope.

20. The judgme?it of solidity. The stereoscope.

LESSON XI.

The Nervous System and Innervation.
Pp. 278-303.

§ I. The nervous system.

2. The cerebro-spinal a?td sympathetic systems.

3. The menibrajies of the cerebro-spinal axis.

4. The spinal cord. The roots of the spinal nerves.

5. Tra7isverse section of the spi?ial cord j the -white

and grey matter.

6. Physiological properties of nerves. Irritation.

7, 8. The anterior roots of the spinal nerves, motor j the
posteriar, sensory.
9, 10. Molecular changes in a nerve when irritatcd^
Propagation of an impulse.
I T. Properties of the spinal co?d. Conduction of afferent
and efferent i^npulses.
b

viii CONTENTS.

§ 12. Reflex action tJirougli tlic spinal cord.

13. One affe7'e)it nerve may affect^ ilu-oii^h reflex

acfio?i, several efferent nerves. Characters of
reflex actions.

14. Paths of conductio7i of efferent and afferent im-

pulses along the spinal cord.
, 1 5. Vaso-niotor centres.

16. The brain ; the outlines of its anatomy.

17. The a7'rangeme7it of its white a7id grey 77} alter.

1 8. The 77C7'ves give7i offfro77i the hrai7i.

1 9. TJie olfacto7y and optic 7ie7'7'es i7i 7'eality p7'ocesses

of the b7'ai7i.

20. Effect of i7ijuries to the 77iediilla oblongata.

21. The crossi?tg of efferent i77ipiilses i?t the 7nedulla

oblongata.

22. 77/6' functions of different parts of the brain.

J7itelligence and Will reside i7i the cerebral
he77nspheres.

23. Localisatio7i of fu7ictio7i i/i the ce7'ebral he 771 i-

spheres.

24. Reflex actio7i takes place even ivhe7i the brai7i is

whole a7id sou7id.

25. Ma72y ordinary and ve7y co77iplicated 77U(scular

acts are 77iere reflex processes.

26. Artificial reflex actiotis. Educatio7i.

27. The sy77ipathetic syste77i.

LESSON XII.

Histology, or the Minute Structure of the
Tissues. Pp. 304 — 363.

§ I . TJie 77iicroscopical analysis of the body.

2. The body co77iposed of extre7nely 77iinute si77iilar

pa7'ts.

3. A tissue is a 77iultiple of 77iiniite tmits.

4. The tissues are pri77iitively aggregates of nucleated

cells.

5. The divisi07i of the 07>u77i into 7iucleated cells.

6. The succeeding differe7itiaiio7i of these cells. The

chief tissues.

• CONTENTS. xix

§ 7. EpitlicUdl tissue^ epidermis.

8. The structure of epide?'nns.

9. The shedding of the epidermis.

10. The epidermis consists of cells.

11. The growth of the epidermis.

12. The size of the epidermic cells.

13. The glands of the skin.

14. Hairs and nails.

15. 77ie structure of a nail.

16. llie structure of a hair.

17. The epithelium of mucous membranes.

1 8. TJie tissues possessing an intercellular mati'ix.

19. Cartilage.

20. Minute structure of cartilage.

2 1 . Growth a?id development of cartilage.
11. Connective tissue.

23. Varieties of connective tissue.

24. Developme7it of coiinective tissue.

25. General structure of a bone.

26. Bone cojisists of collagenous and calcareous sub-

stances.

27. Minute structure of bo?ie.

28. Nutrition of bone.

29. Developmeiit of bo?ie.

30. Defital tissues : structure of teeth.

31. De7itine^ ejiamel.^ and cement.

32. Development of the teeth.

33. De7itition.

34. Muscle. Getieral structure of a muscle.

35. Structure of a niuscular fibre.

36. Development of a tr.uscular fibre,
yj. Properties of niuscular fibres.

38. Non-striated muscular tissue.

39. Cardiac muscular tissue.

40. Nervoics tissue. Structure of a nerve.
4T. Structure of nerve fibres.

42. Structure of nerve cells in anterior cor?iu.

43. Structure of nerve cells of spi?ial ganglia.

44. Non-mcdullated nerve fibres.

45. Spinal cord. Brain. Olfactory and optic nerves.

XX CONTENTS.

APPENDIX.

Table of Anatomical' and Physiological
Constants. Pp. 365 — 370.

I. General statistics. \. Cutaneous excretions.

II. Digestion. VI. Renal excretion.

III. Circulation. VII. Ncri 'ous action.

\\. Re spiral io?i. ^TII. Histology.

LESSONS

ELEMENTARY PHYSIOLOGY.

LESSON I.

A GENERAL VIEl^V OF THE STRUCTURE AND
FUNCTIONS OF THE HUMAN BODY.

I. The body of a living' man performs a great diversity
of actions, some of which are quite obvious ; others re-
quire more or less careful observation ; and yet others can
be detected only by the employment of the most delicate
appliances of science.

Thus, some part of the body of a living- man is plainly
always in motion. Even in sleep, when the limbs, head,
and eyelids may be still, the incessant rise and fall of the
chest continue to remind us that we are viewing slumber
and not death.

More careful observation, however, is needed to detect
the motion of the heart ; or the pulsation of the arteries ;
or the changes in the size of the pupil of the eye with vary-
ing light ; or to ascertain that the air which is breathed out
of the body is hotter and damper than the air which is taken
in by breathing.

And lastly, when we try to ascertain what happens in
the eye when that organ is adjusted to different distances :

13 B

2 ELEMENTARY niVSIOLOGY. [les^.

or what in a nerve when it is excited : or of what materials
flesh and blood are made : or in virtue of what mechanism
it is that a sudden pain makes one start — we have to call
into operation all the methods of inductive and deductive
logic ; all the resources of physics and chemistr)' ; and all
the delicacies of the art of experiment.

2. The sum of the facts and generalizations at which we
arrive by these various modes of inquiry, be they simple
or be they refined, concerning the actions of the body and
the manner in which those actions are brought about, con-
stitutes the science of Human Physiology. An elementary
outline of this science, and of so much anatomy as is inci-
dentally necessary, is the subject of the following Lessons ;
of which I shall devote the present to an account of so
much of the structure and such of the actions (or, as they
are technically called, " functions " ) of the body, as can be
ascertained by easy observation ; or might be so ascer-
tained if the bodies of men were as easily procured, exa-
mined, and subjected to experiment, as those of animals.

3. Suppose a chamber with walls of ice, through which
a current of pure ice-cold air passes ; the walls of the
chamber will of course remain unmelted.

Now, having weighed a healthy living man with great
care, let him walk up and down the chamber for an hour.
In doing this he will obviously exercise a great amount of
mechanical force ; as much, at least, as would be required
to lift his weight as high and as often as he has raised him-
self at ever)' step. But, in addition, a certain quantity of the
ice will be melted, or converted into water ; showing that
the man has given offbeat in abundance. Furthermore, if
the air which enters the chamber be made to pass through
lime-water, it will cause no cloudy white precipitate of
carbonate of lime, because the quantity of carbonic acid
in ordinary air is so small as to be inappreciable in this
way. But if the air which passes out is made to take the
same course, the lime-water will soon become milky, from
the precipitation of carbonate of lime, showing the pre-
sence of carbonic acid, which, like the heat, is given off by
the man.

Again, even if the air be quite dry as it enters the cham-
ber (and the chamber be lined with some material so as to
shut out all vapour from the melting ice walls), that which

I.J WORK AND WASTE. 3

is breathed out of the man, and that which is given otf
from his skin, will exhibit clouds of vapour ; which vapour,
therefore, is derived from the body.

After the expiration of the hour during which the expe-
riment has lasted, let the man be released and weighed
once more. He will be found to have lost weight.

Thus a living, active man, constantly exerts mechajiical
force^ gives off heat^ evolves carbolic acid and water, and
undergoes a loss of substance.

4, Plainly, this state of things could not continue for
an unlimited period, or the man would dwindle to nothing.
But long before the effects of this gradual diminution of
substance become apparent to a bystander, they are felt
by the subject of the experiment in the form of the two
imperious sensations called hunger and thirst. To still
these cravings, to restore the weight of the body to its
former amount, to enable it to continue giving out heat,
water, and carbonic acid, at the same rate, for an indefinite
period, it is absolutely necessary that the body should be
supplied with each of three things, and with three only.
These are, firstly, fresh air ; secondly, drink^consisting
of water in some shape or other, however much it may be
adulterated ; thirdly, food. That compound known to
chemists as proteid matter, and which contains carbon,
hydrogen, oxygen, and nitrogen, must form a part of this
food, if it is to sustain life indefinitely ; and fatty, starchy,
or saccharine matters ought to be contained in the food,
if it is to sustain life conveniently.

5. A certain proportion of the matter taken in as food
either cannot be, or at any rate is not, used ; and leaves
the body, as excrei)ie?ititious matter, having simply passed
through the alimentan,- canal without undergoing much
change, and without ever being incorporated into the
actual substance of the body. But, under healthy con-
ditions, and when only so much food as is necessaiy is
taken, no important proportion of either proteid matter,
or fat, or starchy or saccharine food, passes out of the
body as such. Almost all real food leaves the body in
the form either of water, or of carbonic acid, or of a third
substance called itrea, or of certain saline compounds.

Chemists have determined that these products which are
thrown out of the body and are called excretions, contain

B 2

4 ELEMENTARY PitVSlOLOGY. [less.

if taken altogether, far more oxygen than the food and
water taken into the body. Now, the only possible source
whence the body can obtain oxygen, except from food and
water, is the air which surrounds it.^ And careful inves-
tigation of the air which leaves the chamber in the imagi-
nary experiment described above would show, not only
that it has gained carbonic acid/r^w the man, but that it
has lost oxygen in equal or rather greater amount to him.

6. Thus, if a man is neither gaining nor losing weight,
the sum of the weights of all the substances above enume-
rated whi:h leave the body ought to be exactly equal to
the weight of the food and water which enter it, together
with that of the oxygen which it absorbs from the air.
And this is proved to be the case.

Hence it follows that a man in health, and " neither
gaining nor losing flesh," is incessantly oxidating and
wasting away, and periodically making good the loss.
So that if, in his average condition, he could be confined
in the scale-pan of a delicate spring balance, like that
used for weighing letters, the scale-pan would descend at
every meal, and ascend in the intervals, oscillating to
equal distances on each side of the average position,
which would never be maintained for longer than a few
minutes. There is, therefore, no such thing as a sta-
tionary^ condition of the weight of the body, and what we
call such is simply a condition of variation within narrow
limits — a condition in which the gains and losses of the
numerous daily transactions of the economy balance one
another.

7. Suppose this diurnally-balanced physiological state
to be reached, it can be maintained only so long as the
quantity of the mechanical work done, and of heat, or
other force evolved, remains absolutely unchanged.

Let such a physiologically-balanced man lift a heavy
body from the ground, and the loss of weight which he
would have undergone without that exertion will be
immediately increased by a definite amount, which
cannot be made good unless a proportionate amount of

' Fresh country air contains in every 100 parts nearly 21 of oxygen and
79 of nitrogen gas, together with a small fraction of a part of carbonic acid,
a minute uncertain proportion of ammonia, and a variable quantity of water j*
vapour. (See Lesson IV. §11.)

I.] THE BUILD OF THE llODY. 5

extra food be supplied to him. Let the temperature of
the air fall, and the same result will occur, if his body
remains as warm as before.

On the other hand, diminish his exertion and lower his
production of heat, and either he will gain weight, or
some of his food will remain unused.

Thus, in a properly nourished man, a stream of food is
constantly entering the body in the shape of complex
compounds containing comparatively little oxygen ; as
constantly, the elements of the food (whether before or
after they have formed part of the living substance) are
leaving the body, combined with more oxygen. And the
incessant breaking down and oxidation of the complex
compounds which enter the body are definitely propor-
tioned to the amount of energy the body gives out, whether
in the shape of heat or otherwise ; just in the same way as
the amount of work to be got out of a steam-engine, and
the amount of heat it and its furnace give off, bear a strict
proportion to its consumption of fuel.

8. From these general considerations regarding the
nature of life, considered as physiological work, we
may turn for the purpose of taking a like broad survey
of the apparatus which does the work. We have seen
the general performance of the engine, we may now look
at its build.

The human body is obviously separable into head^
trujik^ and li))ibs. In the head, the brain-case or skull
is distinguishable from the face. The trunk is naturally
divided into the chest or thorax^ and the belly or abdo-
men. Of the hmbs there are two pairs — the upper, or
arms, and the lower, or legs ; and legs and arms again
are subdivided by their joints into parts which obviously
exhibit a rough correspondence — tJiigh and upper anu^
leg and fore-arm, a?ikle and lurist, fingers and toes^
plainly answering to one another. And the two last, in
fact, are so similar that they receive the same name of
digits ; while the several joints of the fingers and toes
have the common denomination of phala?iges.

The whole body thus composed (without the viscera) is
seen to be bilaterally symmetrical ; that is to say, if it
were split lengthways by a great knife, which should be
made to pass along the middle line of both the dorsal and

6 ELEMKNTARV FlIVSTOLOGY. [less.

ventral (or back and front) aspects, the two halves would
almost exactly resemble one another,

9. One-half of the body, divided in the manner de-
scribed (Fig. I, A), would exhibit in the trunk, the
cut faces of thirty-three bones, joined together by a
very strong and tough substance into a long column,
which lies much nearer the dorsal (or back) than the
ventral (or front) aspect of the body. The bones thus
cut through are called the bodies of the vertcbrce. They
separate a long, narrow canal, called the spinal canal,
which is placed upon their dorsal side, from the spacious
chamber of the chest and abdomen, which lies upon their
ventral side. There is no direct communication between
the dorsal canal and the ventral cavity.

The spinal canal contains a long white cord — the spinal
cord — which is an important part of the nervous system.
The ventral chamber is divided into the two subordinate
cavities of the thorax and abdomen by a remarkable,
partly fleshy and partly membranous, partition, the dia-
phragm (Fig. I, D), which is concave towards the abdo-
men, and convex towards the thorax. The alinicnta?y
canal {Fig. i, Al.) traverses these cavities from one end to
the other, piercing the diaphragm. So does a long double
series of distinct masses of nervous substance, which are
called ga)iglia, are connected together by nervous cords,
and constitute the so-called synnpathetic (Fig. i, Sy). The
abdomen contains, in addition to these parts, the two
kidneys, one placed against each side of the vertebral
column, the liver, the pancreas or " sweetbread " and the
spleen. The thorax incloses, besides its segment of the
alimentary canal and of the sympathetic, the heart and
the two lungs. The latter are placed one on each side of
the heart, which lies nearly in the middle of the thorax.

Where the body is succeeded by the head, the upper-
most of the thirty-three vertebral bodies is followed by a
continuous mass of bone, which extends through the whole
length of the head, and, like the spinal column, separates
a dorsal chamber from a ventral one. The dorsal cham-
ber, or cavity of the skull, opens into the spinal canal. It
contains a mass of nervous matter called the brain,\\\\\c\\
is continuous with the spinal cord, the brain and the spinal
cord together constituting what is termed the cerebrospinal

I.l

THE TISSUES.

Fig. I.

A. A diagrammatic secticn of the human body taken vertically through the
median plane. C.S. the cerebro-sp.nal nervous system; A, the cavity of the
ncse ; M, that of the m-:uth ; A/. Al. the alimentarj- canal represented as a
simple straight tub-; H, the heart; D, the diaphragm; Sy, the sympathetic
ganglia. . i-

B. A transverse vertical section of the head taken along the line a b ; letters
as before.

C. A transverse section taken along the line c d ; letters as before.

8 FXKMENTARV rilVSIOLOGV. [less.

axis (Fig. C.S., C.S.). The ventral chamber, or cavity
of the face, is ahiiost entirely occupied by the mmith and
pharynx^ into which last the upper end of the alimentary
canal (called gullet or a'sophagus) opens.

10. Thus, the study of a longitudmal section shows us
that the human body is a double tube, the two tubes being
completely separated by the spinal column and the bony
axis of the skull, which form the floor of the one tube and
the roof of the other. The dorsal tube contains the cere-
bro-spinal axis ; the ventral tube contains the alimentary
canal, the sympathetic nervous system, the heart, and the
lungs, besides other organs.

Transverse sections, taken perpendicularly to the axis
of the vertebral column, or to that of the skull, show still
more clearly that this is the fundamental structure of the
human body, and that the great apparent difference be-
tween the head and the trunk is due to the different size
of the dorsal cavity relatively to the ventral. In the head
the former cavity is very large in proportion to the size of
the latter (Fig. i, B) ; in the thorax, or abdomen it is very
small (Fig. i, C).

The limbs contain no such chambers as are found in
the body and the head ; but with the exception of certain
branching tubes filled with fluid, which are called blood-
7'essels and lymphatics, are solid or semi-solid, throughout.

11. Such being the general character and arrangement
of the parts of the human body, it will next be well to con-
sider into what constituents it may be separated by the
aid of no better means of discrimination than the eye and
the anatomist's knife.

With no more elaborate aids than these, it becomes
easy to separate that tough membrane which invests the
whole body, and is called the skin, or i>iteguj?ie?it, from the
parts which lie beneath it. Furthermore, it is readily
enough ascertained that this integument consists of two
portions : a superficial layer, which is constantly being
shed in the form of powder or scales composed of minute
particles of horny matter, and is called the epidermis;
and the deeper part, the dermis, \\hich is dense and fibrous
(Fig. 32). The epidermis, if wounded, neither gives rise
to pain nor bleeds. The dermis, under like circum-
stances, is ver)' tender, and bleeds freely. A practical

i.l THE TISSUES. o

distinction is drawn between the two in shaving, in the
course of which operation the razor ought to cut onlv
epidermic structures ; for if it go a shade deeper, it gives
rise to pain and bleeding.

The skin can be readily enough removed from all parts
of the exterior, but at the margins of the apenures of the
body it seems to stop, and to be replaced by a layer which
is much redder, more sensitive, bleeds more readily, and
which keeps itself continually moist by giving out a more
or less tenacious fluid, called yiiucus. Hence, at these
apertures, the skin is said to stop, and to be replaced by
mucous niemdram, which lines all those interior ca\-ities,
such as the alimentary- canal, into which the apertirres
open. But, in truth, the skin does not really come to an
end at these points, but is directly continued into the
mucous membrane, which last is simply an integument of
greater delicacy, but consisting fundamentally of the same
two layers, — a deep, fibrous layer, containing blood-vessels,
and a superficial bloodless one, now called the epitheliujtt.
Thus ever}- part of the body might be said to be con-
tained between the walls of a double bag. formed by the
epidermis, which invests the outside of the body, and
the epithelium, its continuation, which lines the alimentary
canal and similar cavities.

12. The dermis, and the deep, sanguine layer, which
answers to it in the macous membranes, are chiefly made
up of a filamentous substance, which yields abundant
gelatine on being boiled, and is the matter which tans
when hide is made into leather. This is called areolar^
fibrous^ or, better, connective tissue.^ The last name is the
best, because this tissue is the great connecting medium
by which the dinerent parts of the body are held together.
Thus it passes from the dermis between all the other
organs, ensheathing the muscles, coating the bones and
cartilages, and eventually reaching and entering into the
mucous membranes. And so completely and thoroughly
does the connective tissue permeate almost all parts of
the body, that if even.- other tissue could be dissected
away, a complete model of all the organs would be left
composed of this tissue. Connective tissue varies very

^ Even* such constituent of the body, as epidermis, cartilage, or muscle,
is called a "* tissue." (See Lesson XII.)

ro ET.1:MI:NTARY PHVSIOI.OrrV. [lesr.

much in character ; in some places being very soft and
tender, at others — as in the tendons and hgaments, which
are ahnost wholly composed of it — attaining great strength
and density.

13. Among the most important of the tissues imbedded
in and ensheathed by the connective tissue, are some the
presence and action of which can be readily determined
during life.

If the upper arm of a man whose arm is stretched out
be tightly grasped by another person, the latter, as the
former bends up his fore-arm, will feel a great soft mass
which lies at the fore part of the upper arm, swell, harden,
and become prominent. As the arm is extended again,
the swelling and hardness vanish.

On removing the skin, the body which thus changes its
configuration is found to be a mass of red flesh, sheathed
in connective tissue. The sheath is continued at each end
into a tendon, by which the muscle is attached, on the
one hand, to the shoulder-bone, and, on the other, to one
of the bones of the fore-arm. This mass of flesh is the
muscle called biceps^ and it has the peculiar property of
changing its dimensions — shortening and becoming thick
in proportion to its decrease in length — when influenced
by the will as well as by some other causes,^ and of
returning to its original form when let alone This
temporary change in the dimensions of a muscle, this
shortening and becoming thick, is spoken of as its C07i-
traction. It is by reason of this property that muscular
tissue becomes the great motor agent of the body ; the
muscles being so disposed between the systems of levers
which support the body, that their contraction necessitates
the motion of one lever upon another.

14. These levers form part of the system of hard
tissues which constitute the skeleton. The less hard of
these are the cartilages^ composed of a dense, firm sub-
stance, ordinarily known as " gristle." The harder are the
boties, which are masses of tissue allied to cartilage, or to
connective tissue, hardened by being impregnated with
phosphate and carbonate of lime. They are animal tissues
which have become, in a manner, naturally petrified ; and
when the salts of lime are extracted, as they may be, by

' Such causes are called stiimili.

I.] TTIK SKKT.F.TON. ii

the action of acids, a model of the bone in soft and
llexible animal matter remains.

More than 200 separate bones are ordinarily reckoned
in the human body, though the actual number of distinct
bones varies at different periods of life, many bones which
are separate in youth becoming united together in old age.
Thus there are originally, as we have seen, thirty-three
separate bodies of vertebra: in the spinal column, and the
upper twent)'-four of these commonly remain distinct
throughout life. But the twenty-fifth, twenty-sixth, twenty-
seventh, twenty-eighth, and twenty-ninth early unite into
one great bone, called the sacrtun ; and the four remain-
ing vertebrae often run into one bony mass called the
coccyx'. In early adult life, the skull contains twenty-two
naturally separate bones, but in youth the number is
much greater, and in old age far less. Twenty-four ribs
bound the chest laterally, twelve on each side, and most
of them are connected by cartilages with the breast-bone.
In the girdle which supports the shoulder, two bones are
always distinguishable as the scapula and the clavicle.
The pdz't's^ to which the legs are attached, consists of two
separate bones called the ossa imiominata in the adult ;
but each os mno))iinatii])i is separable into three (called
pubis, ischium, and ilium) in the young. There are thirty
bones in each of the arms, and the same number in each
of the legs, counting the patella, or knee pan.

All these bones are fastened together by ligaments, or
by cartilages ; and where they play freely over one
another, a coat of cartilage furnishes the surfaces which
come into contact. The cartilages which thus form part
of a joint are called articular cartilages, and their free
surfaces, by which they rub against each other, are lined
by a delicate sy?wvial membrane, which secretes a lubri-
cating fluid — the synovia.

15. Though the bones of the skeleton are all strongly
enough connected together by ligaments and cartilages,
the joints play so freely, and the centre of gravity of the
body, when erect, is so high up, that it is impossible to
make a skeleton or a dead body support itself in the
upright position. That position, easy as it seems, is the
result of the contraction of a multitude of muscles which
oppose and balance one another. Thus, the foot affording

ELEMENTARY rilYSIOLOGY.

[less.

the surface of support, the muscles of the calf (Fig. 2, I)
must contract, or the legs and body would fall forward.

■)

/ /

0 I

Fig. 2. — A Diagram ilm^strating the Attachments of some of thk

MOST IMPORTANT Mr'^CI-ES WHICH KEEP THE BoDY IM THE ErECT

Posture.

T. The muscles of the calf. II. Those of the back of the thigh. III.
Those of the spine. These tend to keep the body from falling forward.

I. The muscles of the front of the leg. 2 Those of the front of the thigh.
3. Those of the front of the abdomen. 4, 5 Those of the front of the neck.
These tend to keep the body from falling backwards. The arrows indicate
the direction of action of the muscles, the foot being fixed.

I.J THE COMBINATION OF ACTIONS. 13

But this action tends to bend the leg ; and to neutralize
this and keep the leg straight, the great muscles in front
of the thigh (Fig. 2, 2) must come into play. But these,
by the same action, tend to bend the body forward on the
legs ; and if the body is to be kept straight, they must be
neutralized by the action of the muscles of the buttocks
and of the back (Fig. 2, III).

The erect position, then, which we assume so easily and
without thinking about it, is the result of the combined
and accurately proportioned action of a vast number af
muscles. What is it that makes them work together in
this w-ay .-'

16. Let any person in the erect position receive a
violent blow on the head, and )ou know what occurs. On
the instant he drops prostrate, in a heap, with his limbs
relaxed and powerless. What has happened to him f
The blow may have been so inflicted as not to touch a
single muscle of the body ; it may not cause the loss of
a drop of blood :. and, indeed, if the " concussion," as it
is called, has not been too severe, the sufferer, after a few
moments of unconsciousness, will come to himself, and
be as well as ever again. Clearly, therefore, no per-
manent injury has been done to any part of the body,
least of all to the muscles, but an influence has been
exerted upon a something which governs the muscles.
And a similar influence may be the effect of very subtle
causes. A strong mental emotion, and even a very bad
smell, will, in some people, produce the same effect as a
blow.

These observations might lead to the conclusion that it
is the mind which directly governs the muscles, but a
little further inquiry will show that such is not the case.
For people have been so stabbed, or shot in the back, as
to cut the spinal cord, without any considerable injury to
other parts : and then they have lost the power of stand-
ing upright as much as before, though their minds may
have remained perfectly clear. And not only have they
lost the power of standing upright under these circum-
stances, but they no longer retain any power of either
feeling what is going on in their legs, or, by an act of
their volition, causing motion in them.

17. And yet, though the mind is thus cut off from the

f4 ELKMKNTARV PIIVSIOLOGV. [less.

lower limbs, a controlling and governing power over them
still remains in the body. For if the soles of the disabled
feet be tickled, though the mind does not feel the tickling,
the legs will be jerked up, just as would be the case in an
uninjured person. Again, if a series of galvanic shocks
be sent along the spinal cord, the legs will perform move-
ments even more powerful than those which the will could
produce in an uninjured person. And, finally, if the injury
is of such a nature as not simply to divide or injure the
spinal cord in one place only, but to crush or profoundly
disorganise it altogether, all these phenomena cease ;
tickling the soles, or sending galvanic shocks along the
spine, will produce no effect upon the legs.

By examinations of this kind carried still further, we
arrive at the remarkable result that while the brain is the
seat of all sensation and mental action, and the primary
source of all voluntary muscular contractions, the spinal
cord is by itself capable of receiving an impression from
the exterior, and converting it not only into a simple
muscular contraction, but into a combination of such
actions.

Thus, in general terms, we may say of the cerebn)-
spinal nervous centres, that they have the power, when
they receive certain impressions from without, of giving
rise to simple or combined muscular contractions.

1 8. But you will further note that these impressions
from without are of very different characters. Any part
of the surface of the body may be so affected as to gi\e
rise to the sensations of contact, or of heat or cold ; and
any or every substance is able, under certain circum-
stances, to produce these sensations. But only very few
and comparatively small portions of the bodily frame-
work are competent to be affected, in such a manner as to
cause the sensations of taste or of smell, of sight or of
hearing : and only a few substances, or particular kinds
of vibrations, are able so to aftect those regions. These
very limited parts of the body, which put us in relation
with particular kinds of substances, or fomis of force,
are what are termed sensory ort^ans. There are two such
organs for sight, two for hearing, two for smell, and one,
or more strictly speaking two, for taste.

19. And now that we have taken this brief view of the

1.1 THE ORGANS. 15

sliiu lure of ihc body, of the organs wliicli support it,
of the ori;ans which move it, and of the or<^ans which
put it in relation with the suirouiuUnL; world, or, in other
words, enable it to move in harmony with inlluences from
without, we must consider the means by which all this
wonderful apparatus is kept in working order.

All work, as we have seen, implies waste. The work
of the nervous S)stem and that of the muscles, therefore,
implies consumption either of their own substance, or of
something else. And as the organism can make nothing,
it must possess the means of obtaining from without that
which it wants, and of throwing off from itself that which
it wastes ; and wc have seen that, in the gross, it does
these things. The bod)- feeds, and it excretes. Hut wc
must now pass from the broad fact to the mechanism
by which the fact is brought about. The organs which
convert food into nutriment are the organs of alimentation /
those which distribute nutriment all over the body arc
organs of circulatioii; those which get rid of the waste
j)roducts are organs of cxoctioi.

20. The organs of alimentation are the mouth, pharynx,
gullet, stomach, and intestines, with their appendages.
What they do is, first to receive and grind the food.
They then act upon it with chemical agents, of which
they possess a store which is renewed as fast as it is
wasted ; and in this way separate the food into a lluid
containing nutritious matters in solution or suspension,
and innutritious dregs ^dx faxes.

21. A system of minute tubes, with very thin walls,
termed capillaries^ is distributed through the whole or-
ganism except the epidermis and its ])roducts, the ej)ithe-
lium, the cartilages, and the substance of the teeth. On
all sides, these tubes pass into others, which are called
arteries and veins; while these, becoming larger and
larger, at length open into the hearty an organ which, as
we have seen, is placed in the thorax. During life,
these tubes and the chambers of the heart, with which
they are connected, are all full of liquid, which is, for
the most part, that red fluid with which we are all familiar
as blood.

The walls of the heart are muscular, and contract
rhythmically, or at regular intervals. By means of these

i6 ELEMENTARY PHYSIOLOGY. [less.

contractions the blood which its cavities contain is driven
in jets out of these cavities, into the arteries, and thence
into the capillaries, whence it returns by the veins back
into the heart.

This is the circulation of the blood.

22. Now the fluid containing the dissolved or suspended
nutritive matters which are the result of the process of
digestion, traverses the very thin layer of soft and per-
meable tissue which separates the cavity of the alimentary
canal from the cavities of the innumerable capillary vessels
which lie in the walls of that canal, and so enters the
blood, with which those capillaries are filled. Whirled
away by the torrent of the circulation, the blood, thus
charged with nutritive matter, enters the heart, and is
thence propelled into the organs of the body. To these
organs it supplies the nutriment with which it is charged ;
from them it takes their waste products, and, finally,
returns by the veins, loaded with useless and injurious
excretions, which sooner or later take the form of water,
carbonic acid, and urea.

23. These excretionary matters are separated from the
blood by the excretory organs^ of which there are three —
the ski7t, the lungs, and the kidneys.

Different as these organs may be in appearance, they
are constructed upon one and the same principle. Each,
in ultimate analysis, consists of a very thin sheet of tissue,
like so much delicate blotting-paper, the one face of which
is free, or lines a cavity in communication with the
exterior of the body, while the other is in contact with
the blood which has to be purified.

The excreted matters are, as it were (though, as we
shall see, in a peculiar way), strained from the blood,
through thjs delicate layer of filtering-tissue, and on to
its free surface, whence they make their escape.

Each of these organs is especially concerned in the
elimination of one of the chief waste products — water,
carbonic acid, and urea— though it may at the same time
be a means of escape for the others. Thus the lungs are
especially busied in getting rid of carbonic acid, but at
the same time they give off a good deal of water. The
duty of the kidneys is to excrete urea (together with other
saline matters), but at the same time they pass away a

I.] EXCRETION AND OXIDATION. 17

large quantity of water and a trifling amount of carbonic
acid ; while the skin gives off much water, some amount
of carbonic acid, and a certain quantity of saline matter,
among which urea may be, sometimes at all events, present.

24. Finally the lungs play a double part, being not
merely eliminators of waste, or excretionary products,
but importers into the economy of a substance which is
not exactly either food or drink, but something as im-
portant as either^ — to wit, o.x-ygcn.

As the carbonic acid (and water) is passing from the
blood through the lungs into the external air, oxygen is
passing from the air through the lungs into the blood,
and is carried, as we shall see, by the blood to all parts. of
the body. We have seen (p. 5) that the waste which
leaves the body contains more oxygen than the food which
enters the body. Indeed oxidation, the oxygen being
supplied by the blood, is going on all over the body.
All parts of the body are continually being oxidized, or,
in other words, are continually burning, some more
rapidly and fiercely than others. And this burning,
though it is carried on in a peculiar manner, so as never
to give rise to a flame, yet nevertheless produces an
amount of heat which is as efficient as a fire to raise the
blood to a temperature of about ico' ; and this hot fluid,
incessantly renewed in all parts of the economy by the
torrent of the circulation, warms the body, as a house is
warmed by a hot-water apparatus. Nor is it alone the
heat of the body which is provided by this oxidation ;
the energ>' which appears in the muscular work done by
the body has the same source. Just as the burning of the
coal in a steam-engine supplies the motive power which
drives the wheels, so, though in a peculiar way, the
oxidation of the muscles (and thus ultimately of the
food) supplies the motive power of those muscular con-
tractions which carr>' out the movements of the body.
The Food, like coal combustible or capable of oxidation,
is built up into the living body, which in like manner
combustible, is continually being oxidized by the oxygen
of the blood, thus doing work and giving out heat.
Some of the food perhaps may be oxidized without ever
actually forming part of the body or after it has already
become waste matter, but this does not concern us now.

tS F.LFMKNTARV physiology. [less.

25. These alimentary, distributive or circulatory, ex-
cretoiy, and combustive processes would however be worse
than useless if they were not kept in strict proportion one
to another. If the state of physiological balance is to be
maintained, not only must the quantity of aliment taken
be at least equivalent to the quantity of matter excreted ;
but that aliment must be distributed with due rapidity to
the seat of each local waste. The circulatory system is
the commissariat of the physiological army.

Again, if the body is to be maintained at a tolerably
even temperature, while that of the air is constantly vary-
ing, the condition of the hot-water apparatus must be
most carefully regulated.

In other words, a combining organ must be added to
the organs already mentioned, and this is found in the
nerv'ous system, which not only possesses the function al-
ready described of enabling us to move our bodies and to
know what is going on in the external world ; but makes
us aware of the need of food, enables us to discriminate
nutritious from innutritious matters, and to exert the
muscular actions needful for seizing, killing, and cooking ;
guides the hand to the mouth, and governs all the move-
ments of the jaws and of the alimentary canal. By it,
the working of the heart is properly adjusted and the
calibres of the distributing pipes are regulated, so as in-
directly to govern the excretory and combustive processes.
And these are also more directly affected by other actions
of the ner\ous system.

26. The various functions which have been thus briefly
indicated constitute the greater part of what are called
the vital actions of the human body, and so long as they
are performed, the body is said to possess life. The ces-
sation of the performance of these functions is what is
ordinarily called death.

But there are really several kinds of death, which may,
in the first place, be distinguished from one another under
the two heads of local and of general death.

27. Local death is going on at every moment, and in
most, if not in all, parts of the living body. Individual
cells of the epidermis and of the epithelium are inces-
santly dying and being cast off, to be replaced by others
which are, as constantly, coming into separate existence.

I.] I.OCAT. AND CxENKRAL D?:ATH. 19

The like is true of blood-corpuscles, and probably of many
other elements of the tissues.

This form of local death is insensible to ourselves, and
is essential to the due maintenance of life. But, occa-
sionally, local death occurs on a larger scale, as the re-
sult of injury, or as the consequence of disease. A burn,
for example, may suddenly kill more or less of the skin ;
or part of the tissues of the skin may die, as in the case
of the slough which lies in the midst of a boil ; or a
whole limb may die, and exhibit the strange phenomena
of uiortijicatioii.

The local death of some tissues is followed by their
regeneration. Not only all the forms of epidermis and
epithelium, but nerves, connective tissue, bone, and at
any rate, some muscles, may be thus reproduced, even on
a large scale.

28. General death is of two kinds, death 0/ the body as
a -uhole, and death of the tissues. By the former term is
implied the absolute cessation of the functions of the
brain, of the circulatory, and of the respiratory organs ;
by the latter, the entire disappearance of the vital actions
of the ultimate structural constituents of the body.
When death takes place, the body, as a whole, dies first,
the death of the tissues not occurring until after an
inter^-al, which is sometimes considerable.

Hence it is that, for some little time after what is ordi-
narily called death, the muscles of an executed criminal
may be made to contract by the application of proper
stimuli. The muscles are not dead, though the man is.

29. The modes in which death is brought about appear
at first sight to be extremely varied. We speak of natural
death byold age, or by some of the endless forms of dis-
ease ; of violent death by starvation, or by the innumer-
able varieties of injury, or poison. But, in reality, the
immediate cause of death is always the stoppage of the
functions of one of three organs ; the cerebro-spinal ner-
vous centre, the lungs, or the heart. Thus, a man may
be instantly killed by such an injury to a part of the brain
which is called the medulla oblo?igata (see Lesson XI.)
as may be produced by hanging, or breaking the neck.

Or death may be the immediate result of suffocation

C 2

20 ELEMENTARY PHVSTOLOGV. [less.

by strangulation, smothering, or drowning, — or, in other
words, of stoppage of the respirator}' functions.

Or, finally, death ensues at once when the heart ceases
to propel blood. These three organs — the brain, the
lungs, and the heart— have been fancifully termed the
tripod of life.

In ultimate analysis, however, life has but two legs to
stand upon, the lungs and the heart, for death through
the brain is always the effect of the secondary- action of
the injury to that organ upon the lungs or the heart. The
functions of the brain cease, when either respiration or
circulation is at an end. But if circulation and respira-
tion are kept up artificially, the brain may be removed
without causing death. On the other hand, if the blood
be not aerated, its circulation by the heart cannot pre-
serA-e life ; and, if the circulation be at an end, mere
aeration of the blood in the lungs is equally ineffectual
for the prevention of death.

30. With the cessation of life, the everyday forces of
the inorganic world no longer remain the servants of the
bodily frame, as they were during life, but become its
masters. Oxygen, the slave of the living organism,
becomes the lord of the dead body. Atom by atom, the
complex molecules of the tissues are taken to pieces and
reduced to simpler and more oxidized substances, until
the soft parts are dissipated chiefly in the form of car-
bonic acid, ammonia, water, and soluble salts, and the
bones and teeth alone remain. But not even these dense
and earthy structures are competent to offer a permanent
resistance to water and air. Sooner or later the animal
basis which holds together the earthy salts decomposes
and dissolves — the solid structures become friable, and
break down into powder. Finally, they dissolve and are
diffused among the waters of the surface of the globe,
just as the gaseous products of decomposition are dis-
sipated through its atmosphere.

It is impossible to follow, with any degree of certainty,
wanderings more varied and more extensive than those
imagined by the ancient sages who held the doctrine of
transmigration ; but the chances are, that sooner or later,
some, if not all, of the scattered atoms \\\\\ be gathered
into new forms of life.

1.] CHANGES OF MATTER. 21

The sun's rays, acting through the vegetable world,
build up some of the wandering molecules of carbonic
acid, of water, of ammonia, and of salts, into the fabric
of plants. The plants are devoured by animals, animals
devour one another, man devours both plants and other
animals ; and hence it is ver)' possible that atoms which
once formed an integral part of the busy brain of Julius
Caesar may now enter into the composition of Caesar the
negro in Alabama, and of Caesar the house-dog in an
English homestead.

And thus there is sober truth in the words which Shake-
speare puts into the mouth of Hamlet—

'■ Imperial Cajsar, dead and turned to clay,
Might stop ah,!:: tj keep the cold away ;
Oh that that ear '.1. which kept the world in awe.
Should patch a wall, t' expel the winter's flaw I "

22 ELEMENTARY PHYSIOLOGY. [less,

LESSON IE

THE VASCULAR SYSTEM AND THE CIRCULATION.

I. Almost all parts of the body are vascular j that is
to say, they are traversed by minute and very close-set
canals, which open into one another so as to constitute a
small-meshed network, and confer upon these parts a
spongy texture. The canals, or rather tubes, are pro-
vided with distinct but very delicate walls, composed of
what at first sight appears to be a structureless membrane
(Fig. 3 A, a), but is in reality formed of a number of thin
scales, called " cells," cemented together at their edges ; in
each of these cells lies a small oval body (Fig. 3 A, ^),
termed a 7iucleus (see Lesson XIL § 2).

These tubes are the capillaries. They vary in diameter
from yyVoth to To^ootl'i of an inch ; they are sometimes
disposed in loops, sometimes in long, sometimes in wide,
sometimes in narrow meshes ; and the diameters of these
meshes, or, in other words, the interspaces between the
capillaries, are sometimes hardly wider than the diameter
of a capillary, sometimes many times as wide (see Figs. 16,
20, 32, 33, and 37). These interspaces are occupied by the
substance of tlic tissue which the capillaries permeate
• Fig. 3 A, c) so that the ultimate anatomical components
of every part of the body are, strictly speaking, outside
the vessels, or extra-i'ascula?:

But there are certain parts which, in another and
broader sense, are also said to be extra-vascular or non-
vascular. These are the epidermis and epithelium, the
nails and hairs, the substance of the teeth, and to a certain
extent the cartilages ; which may and do attain a very
considerable thickness or length, and yet contain no

II.J

CAPILLARIES.

vessels. However, since we have seen that all the tissues
are really extra-vascular, these differ only in degree from
the rest. The circumstance that all the tissues are outside
the vessels by no means interferes with their being bathed
by the fluid which is inside the vessels. In fact, the walls
of the capillaries are so exceedingly thin that their fluid

Fig. 3.

A. Diagrainiiiatic representation of a capillary seen from above and in
section: a. the wall of the capillary with b, the nuclti; c, nuclei belonging
to the connective tissue in which the cai>illary is supposed to be lying ; d, the
canal of the capillary.

H. Diagrammatic representation cf the structure cf a small r.rtery : a,
epithelium : b, the sc-called basement membrane ; r, the circular non-striateil
nuisciilar fibres, each with nuciaus <i ; e, the coat of fibrous tissue with
nuclei /.

contents readily exude through the delicate membrane of
which they are composed, and irrigate the tissues in which
they lie.

2. The capillary tubes thus described contain, during
life, the red fluid, blood. There are other somewhat

24 ELEMENTARY PHYSIOLOGY. [less.

similar tubes, also sometimes called capillaries, but these
are filled v%'ith a pale, watery, or milky fluid, termed
lytnph^ or chyle^ and are called lymphatics. The capil-
laries, which contain blood, are continued on different
sides, into somewhat larger tubes, with thicker walls,
which are the smallest arteries and veins, and these again
join on to larger arteries and veins.

The mere fact that the walls of these vessels are thicker
than those of the capillaries constitutes an important
difference between the capillaries and the small arteries
and veins ; for the walls of the latter are thus rendered
far less permeable to fluids, and that thorough irrigation
of the tissues, which is effected by the capillaries, cannot
be performed by them.

The most important difference between these vessels
and the capillaries, however, lies in the circumstance that
their walls are not only thicker, but also more complex,
being composed of several coats, one, at least, of which
is muscular. The number, arrangement, and even nature
of these coats differ according to the size of the vessels,
and are not the same in the veins as in the arteries, though
the smallest veins and arteries tend to resemble each
other.

If we take one of the smallest arteries, we find, first,
a very delicate lining of cells constituting a sort of epi-
thelium continuous with the cells which form the only
coat of the capillaries (Fig. 3 B, a). Outside this,
separated from it by a thin but strong membrane (shown
as a mere line at Fig. 3 B,(^), comes the muscular coat of
the kind called plain or non-striated muscle (see Lesson
XI L), made up of flattened spindle-shape fibres which
are wrapped round the vessel (Fig. 3 15, c).

Outside the muscular coat is a sheath of fibrous or
connective tissue (Fig. 3 B, /).

In the smallest arteries there is but a single layer of
these muscular fibres encircling the vessel like a series of
rings ; but in the larger arteries there are several layers
of circular muscular fibres variously bound toget.her with
fibrous and elastic tissue, though as the vessels get larger
the quantity of muscular tissue in them gets relatively
less.

Now these plain muscular fibres possess that same

II.] PROPERTIES OF ARTERIES. 25

power of contraction, or shortening in the long, and
broadening in the narrow, directions, which, as was stated
in the preceding Lesson, is the special property of mus-
cular tissue. And when they exercise this power, they,
of course, narrow the calibre of the vessel, just as squeezing
it with the hand or in any other way would do ; and this
contraction may go so far as, in some cases, to reduce the
cavity of the vessel almost to nothing, and to render it
practically impervious.

The state of contraction of these muscles of the small
arteries is regulated, like that of other muscles, by
their nerves ; or, in other words, the nerves supplied
to the vessels determine whether the passage through
these tubes should be wide and free, or narrow and ob-
structed. Thus while the small arteries lose the function,
which the capillaries possess, of directly irrigating the
tissues by transudation, they gain that of regulating the
supply of fluid to the irrigators or capillaries themselves.
The contraction, or dilatation, of the arteries which
supply a set of capillaries, comes to the same result as
lowering or raising the sluice-gates of a system of
irrigation-canals.

3. The smaller arteries and veins severally unite into,
or are branches of, larger arterial or venous trunks, which
again spring from or unite into still larger ones, and these,
at length, communicate by a few principal arterial and
venous trunks with the heart.

The smallest arteries and veins, as we have seen, are
similar in structure, but the larger arteries and veins differ
widely ; for the larger arteries have walls so thick and
stout that they do not sink together when empty ; and
this thickness and stoutness arises from the circumstance
that not only is the muscular coat very thick, but that
in addition, and more especially, several layers of a highly
elastic, strong, fibrous substance become mixed up with
the muscular layers. Thus, when a large a.nery is pulled
out and let go, it stretches and returns to its primitive
dimensions almost like a piece of india-rubber.

The larger veins, on the other hand, contain but little
of either elastic or muscular tissue. Hence, their walls
are thin, and they collapse when empty.

This is one cjreat difference between the larger arteries

ELEMENTARY PHYSIOLOGY.

[less.

and the veins ; the other is the presence of what are
termed valves in a great many of the veins, especially in
those which lie in muscular parts of the body. They are
absent in the largest trunks, and in the smallest branches,
and in all the divisions of the portal, pulmonary, and
cerebral veins.

4. These valves are pouch-like folds of the inner wall
of the vein. The bottom of the pouch is turned towards
those capillaries from which the vein springs. The free
edge of the pouch is directed the other way, or towards
the heart. The action of these pouches is to impede the
passage of any fluid from the heart towards the capillaries,
while they do not interfere with fluid passing in the oppo-
site direction (Fig. 4). The v.orking of some of these
valves maybe very easily demonstrated in the living body.

'//'///I / i/ii II li I '

Fig. 4. -The Valves of Veins.

C, H, C, II, Diagrammatic sections of veins with valves. In the upper

figure the blood is supposed to be flowing in the direction of the arrow,

towards the lieart ; in the lower, back towards the capillaries ; C, capillary

side ; H, heart side. A, a vein laid open to showapairof pouch shaped valves.

When the arm is bared, blue veins may be seen running
from the hand, under the skin, to the upper arm. The
diameter of these veins is pretty even, and diminishes
regularly towards the hand, so long as the current of the
blood, which is running in them, from the hand to the
upper arm, is uninterrupted.

Bi't if a finger be pressed upon the upper part of one
of these veins, and then passed downwards along it, so as
to dri\e the blood which it contains backwards, sundr\-
swellings, like little knots, will suddenly make their ap-
pearance at several points in the length of the vein, where
nothing of the kind was visible before. These swellings
are simply dilatations of the wall of the vein, caused by
the pressure of the blood on that wall, above a valve

II.J

LVMIMIATICS.

which opposes its backward progress. The iiioment the
backward impulse ceases the blood Hows on again ; the
valve, swinging back towards the wall of the vein, affords
no obstacle to its progress, and the distension caused by
its pressure disappears (Fig. 4).

The only arteries which possess
valves are the primary trunks— the
aorta and pulmonary artery — which
spring from the heart, and these
valves, since they really belong to the
heart, will be best considered with
that organ.

5. Besides the capillary network
and the trunks connected with it,
which constitute the blood-vascular
system, all parts of the body which
possess blood capillaries also contain
another set of what are termed lyin-
pJiatic capillaries, mixed up with
those of the blood-vascular system,
but not directly communicating with
them, and, in addition, differing from
the blood capillaries in being con-
nected with larger vessels of only one
kind. That is to say, they open only
into trunks which carry fluid away
from them, there being no large
vessels which bring anything to them.

These trunks further resemble the
small veins in being abundantly pro-
A ided with valves which freely allow
of the passage of liquid from the
lymphatic capillaries, but obstruct the
flow of anything the other way. But
the lymphatic trunks differ from the
veins, in that they do not rapidly unite
into larger and larger trunks, which
present a continually increasing
calibre, and allow of a flow without
heart. On the contrary, remaining
size, they, at intervals, enter and ramify in rounded bodies
called ly'DipJiatk glands, whence new lymphatic trunks

5. — Thk Lvmi'H.\-
Tics OF THE Front of
THE RinHT Arm.

g. Lymphatic glands, or
gangiiii as they are
sometimes called. 'I'hes*-
gaugiia are not to !)<•
canfounded with \\c\-
\ou^ gn»g//ii.

interruption to the
nearly of the same

ELEMENTARY PHYSIOLOGY.

[less.

Fig. 6. — Tnii Thoracic Duct.

The Thoracic Duct occiipies the middle of the figure. It lies upon th ■
spinal column, at the sides cf which are seen portions of the ribs (i).
a, the receptacle of the chyle ; l>, the trunk of the thoracic duct, opening
at c into the junction of the left jugular (/) and subclavian {g) veins as
they unite into the left innominate vein, which lias been cut across to
show the thoracic duct running behind it ; </, lymphatic glands placed in
the lumbar regions; /i, the superior vena cava formed by the junction cf
the right and left innuminale veins.

II.] Till'. THORACIC DUCT. 29

arise (Fig-. 5}. In these glands the lymphatic capil-
laries and passages are closely interlaced with blood
capillaries.

Sooner or later, however, the great majority of the
smaller lymphatic tnm.s pour their contents into a tube,
which is about as large as a crow-quill, lies in front of the
backbone, and is called the tJioracic duct. This opens at
the root of the neck into the conjoined trunks of the great
veins which bring back the blood from the left side of
the head and the left arm (Fig. 6). The remaining
lymphatics are connected by a common canal with the
corresponding \'ein on the right side.

Where the principal trunks of the lymphatic system
open into the veins, valves are placed, which allow of the
passage of fluid in one direction only, viz. from the lym-
phatic to the vein. Thus the lymphatic vessels are, as it
were, a part of the venous system, though, by reason of
these valves, the fluid which is contained in the veins
cannot get into the lymphatics. On the other hand, ever)'
facility is attbrded for the passage into the veins of the
fluid contained in the lymphatics. Indeed, in consequence
of the numerous valves in the lymphatics, every pressure
on their walls, not being able to send the fluid backward,
must drive it more or less forward, towards the veins.

6. The lower part of the thoracic duct is dilated, and
is termed the receptacle^ or cistern^ of the chyle {a, Fig 6).
In fact, it receives the lymphatics of the intestines, which,
though they differ in no essential respect from other lym-
phatics, are called lacteals, because, after a meal con-
taining much fatty matter, they are filled with a milky
fluid, which is termed the chyle. The lacteals, or lym-
phatics of the small intestine, not only form networks in
its walls, but send blind prolongations into the little
velvety processes termed villi, with which the mucous
membrane of that intestine is beset (see Lesson VI.). The
trunks which open into the network lie in the mesejitery
(or membrane which suspends the small intestine to
the back wall of the abdomen), and the glands through
which these trunks lead are hence termed the mesenteric
gla?ids.

7. It will now be desirable to take a general view of the
arrangement of all these difterent vessels, and of their

ELEMENTARY PHYSIOLOGY

[less.

Fig. 7. — Diagram of the Heakt and Vessels, with the Coi'Rse of
THE Circulation, viewed from behind, so that the proper left
OF the Observer corkestonds with the left 5idr of the Heart
IN the Diagram.

L.A. left auricle; L.V. left ventricle ; Ao. aorta; A', arteries to the upper
part of the body ; A^. arteries to the lower part of the body ; JY.A. hepatic
artery, which supplies the liver with part of its blood ; F*. veins of the upper

II.] TTIF. VASCUT.AR SYSTEM. 31

relations to the great rentral ort^^an of the vascular system
— the heart (Fig. 7).

All the veins of ever)- part of the body, except the Kings,
the heart itself, and certain viscera of the abdomen, join
together into larger veins, which, sooner or later, open
into one of two great trimks (Fig. 7, V.C.S. V.C.I.)
termed the superior and the inferior vena cava, which
debouch into the upper or broad end of the right half of
the heart.

All the arteries of every part of the body, except the
lungs, are more or less remote branches of one great
trunk — the aorta (Fig. 7, Ao.), which springs from the
lower division of the left half of the heart.

The arteries of the lungs are branches of a great trunk
(Fig. 7, P. A.) springing from the lower division of the
right side of the heart. The veins of the lungs, on the
contrary, open by four trunks into the upper part of the
left side of the heart (Fig. 7, P. V.).

Thus the venous trunks open into the upper division of
each half of the heart : those of the body in general into
that of the right half, those of the lungs into that of the
left half; while the arterial trunks spring from the lower
moieties of each half of the heart : that for the bod)- in
general from the left side, and that for the lungs from the
right side.

Hence it follows that the great artery of the body, and
the great veins of the body, are connected with opposite
sides of the heart ; and the great arter)^ of the lungs and
the great veins of the lungs also with opposite sides of
that organ. On the other hand, the veins of the body
open into the same side of the heart as the arter)- of the
lungs, and the veins of the lungs open into the same side
of the heart as the artery of the body.

The arteries which open into the capillaries of the sub-
stance of the heart are called coronary arteries, and arise,

part cf the body; V-. veins of the lower part of the body; V.P. vena
portae ; H. V. hepatic vein ; V.C.I, inferior vena cava ; V.C.S. superior vena
cava; R.A. right auricle; R.V. right ventricle: P. A. pulmonarj- arter\- ;
Lg. lung ; P. I', pulmonarj' vein ; Let. lacteals ; Ly. lymphatics : T/i.D.
thoracic duct ; A I. alimentar>' canal ; Lr. liver. The arrows indicate the
Course of the blood, lymph, and chyle. The vessels which contain arterial
blood have dark contours, while those which carry venous blood have light
con'vours.

ELEMENTARY PIIYSIOLOGV.
TV

[LESS.

Fig. 8. — Heart of Sheep, as sesn after Removal from the Body.

LYING upon the TwO LvNGS. ThE PERICARDIUM HAS BEEN CUT
AWAY, BUT NO OTHER DISSECTION MADE.

Ji.A. Auricular appendage of right auricle ; L.A. auricular appendage of
left auricle ; /?. K right ventricle ; L.V. left ventricle ; ^. F.C. superior vena
cava ; I.V.C. inferior vena cava ; /'..<4. pulmonary artery ; Ao, aorta ; A'o',
innominate branch from aorta dividing into subclavian and carotid arteries ;

II.] THE VASCULAR SYSTEM. 33

like the other arteries, from the aorta, but quite close to
its origin, just beyond the semilunar valves. But the
coronary vein, which is formed by the union of the small
veins which arise from the capillaries of the heart, does
not open into either of the venae cavae, but pours the
blood which it contains directly into the division of the
heart into which these veniE cava^ open — that is to say, into
the right upper division (Fig. 14 /;).

The abdominal viscera referred to above, the veins of
which do not take the usual course, are the stomach, the
intestines, the spleen, and the pancreas. These veins all
combine into a single trunk, which is termed the vena
portcE (Fig. 7, V.P.), but this trunk does not open into the
vena cava inferior. On the contrary, having reached the
liver, it enters the substance of that organ, and breaks up
into an immense multitude of capillaries, which ramify
through the liver, and become connected with those into
which the artery of the liver, called the hepatic artery
(Fig, 7, H.A.), branches. From this common capillary
mesh-work veins arise, and unite, at length, into a single
trunk, the hepatic vei?i (Fig. 7, H. V.), which emerges
from the liver, and opens into the inferior vena cava.
The portal vein is the only great vein in the body which
branches out and becomes continuous with the capillaries
of an organ, like an artery.

8. The heart (Figs. 8 and 10), to which all the vessels
in the body have now been directly or indirectly traced,
is an organ, the size of which is usually roughly estimated
as equal to that of the closed fist of the person to whom
it belongs, and which has a broad end turned upwards
and backwards, and rather to the right side, called its
base : and a pointed end which is called its apex, turned
downwards and forwards, and to the left side, so as to lie
opposite the interval between the fifth and sixth ribs.

It is lodged between the lungs, nearer the front than
the back wall of the chest, and is enclosed in a sort of
double bag — the pericardiwn (Fig. 9,/.). One-half of the

L. lung ; Tr. trachea, i, solid cord often present, the remnant of a once open
communication between the pulmonary artery and aorta. 2, masses of fat at
thebases of the ventricle hiding from view the greater part of the auricles.
3, line of fat marking the division between the two ventricles. 4, mass of
fat covering end of trachea.

ELEMENTARY PHYSIOLOGY.

[less.

double bag is closely adherent to the heart itself, forming
a thin coat upon its outer surface. At the base of the heart,
this half of the bag passes on to the great vessels which
spring from, or open into, that organ ; and becomes con-
tinuous with the other half, which loosely envelopes both
the heart and the adherent half of the bag. Between the
two layers of the pericardium, consequently, there is a com-
pletely closed, narrow cavity, lined by an epithelium, and
secreting into its interior a small quantity of clear fluid.^

Fig. 9. — TKA^.-iniv.^c or-^iiox of the Chest, with the Heart and
Lungs in place. (A little diagrammatic.)

D. V. dorsal vertebra, or joint of the backbone ; Ao. Ao' . aorta, the top of
its arch being cut away in this section; S.C. superior vena cava; P. A.
pulmonary- artery, divided into a branch for each lung ; L.P. R.P. left and
right pulmonary veins; Br. bronchi; R.L. L.L. right and left lungs;
Q£. the gullet or oesophagus ;/. outer bag of pericardium ;//. the two layers
of pleura ; v. azygos vein.

The outer layer of the pericardium is firmly connected
below with the upper surface of the diaphragm.

But the heart cannot be said to depend altogether upon
the diaphragm for support, inasmuch as the great vessels

This fluid, like that contained in the peritoneum, pleura, and other shut
sacs of a similar character to the pericardium, used to be called serum ;
whence the membranes forming the walls of these sacs are frequently termed
serous tnenibrancs.

II.]

THE HEART.

which issue from or enter it — and for the most part pass
upwards from its base— help to suspend and keep it in
place.

Thus the heart is coated, outside, by one layer of the
pericardium. Inside, it contains two great cavities or
" divisions," as they have been termed above, completely
separated by a fixed partition which extends from the base
to the apex of the heart ; and consequently, having no

j?.J".'r: ^c X c.

-j:.j.i{

B.ET

Fig. io. — The Heart, Great Vessels, and Ll'ngs. (Front View.)

H.l'. right ventricle; Z.F. left ventricle; J\.A. right auricle; L.A. left
auricle; Ao. aorta; P. A. pulmonary arterj" ; P.f^. pulmonarj- veins;
I^.L. right lung; L.L. left lung; V.S. vena cava superior; S.C. sub-
clavian vessels ; C. carotids ; 7?./. V. and L.J. V. right and left jugular
veins ; /'./. vena cava inferior ; T. trachea ; B. bronchi.

All the great vessels but those of the lungs are cut.

direct communication with one another. Each of these
two great cavities is further subdivided, not longitudinally
but transversely, by a moveable partition. The cavity
above the transverse partition on each side is called the
auricle ; the cavity below, the vejitride — right or left as
the case may be.

D 2

36 ELEMENTARY PHYSIOLOGY. [less.

Each of the four cavities has the same capacity, and is
capable of containing from 4 to 6 cubic inches of water.
The walls of the auricles are much thinner than those
of the ventricles. The wall of the left ventricle is much
thickci- than that of the right ventricle ; but no such
difference is perceptible between the two auricles (Figs.
II and 12, I and 3).

9. In fact, as we shall see, the ventricles have more
work to do than the auricles, and the left ventricle more
to do than the right. Hence the ventricles have more
muscular substance than the auricles, and the left ventricle
than the right ; and it is this excess of muscular substance
ivhich gives rise to the excess of thickness observed in the
left ventricle.

The muscular fibres of the heart are of a peculiar nature,
resembling those of the chief muscles of the body in
being transversely striped (see Lesson XII.), but differing
from them in many other respects.

Almost the whole mass of the heart is made up of
these muscular fibres, which have a very remarkable and
complex arrangement. There is, however, an internal
membranous and epithelial lining, called the e?idoca?'-
diuin ; and at the junction between the auricles and
ventricles, the apertures of communication between their
cavities, called the aurictilo-ventricular apertures, are
strengthened hy fibrous rings. To these rings the move-
able partitions, or valves, between the auricles and
ventricles, the arrangement of which must next be con-
sidered, are attached.

10. There are three of these partitions attached to the
circumference of the right auriculo-ventricular aperture,
and two to that of the left (Figs. 11, 12, 13, 14, /?', mv).
Each is a broad, thin, but very tough and strong trian-
gular fold of connective tissue (see Lesson XII.) covered
by endocardium, attached by its base, which joins on
to its fellow, to the auriculo-ventricular fibrous ring,
and hanging with its point downwards into the ven-
tricular cavity. On the right side there are, therefore,
three of these broad, pointed membranes, whence the
whole apparatus is called the tricuspid valve. On the
left side, there are but two, which, when detached from
all their connexions but the auriculo-ventricular ring, look

11.]

THE VALVES OF THE HEART.

,.s.v-c

—PP

Fig. II. — Right Side of the Heart ok a Sheep.

R.A. cavity of right auricle; S.V.C. superior vena cava; I.V.C. inferior
vena cava ; (a stj'le has been passed through each of these ;) a, a style
passed from the auricle to the ventricle through the auriculo-ventricular
orifice ; b, a style passed into the coronary- vein.

R.V. cavity of right ventricle ; tv, tv, two flaps of the tricuspid valve : the
third is dimly seen behind them, the style a passing between the three.
Between the two flaps, and attached to them by chordce ioidiuetF, is seen a
papillary muscle, />J>, cut away from its attachment to that portion of the
wall of the ventricle which has been removed. Above, the ventricle ter-
minates somewhat like a funnel in the pulmonary arterj^, P. A. One of the
pockets of the semilunar valve, sz>, is seen in its entirety, another partially.

I, the wall of the ventricle cut across ; 2, the position of the auriculo-
ventricular ring ; 3, the wall of the auricle ,\, masses of fat lodged between
the auricle and pulmonarj' artery.

38 ELEMENTARY FHYSiOLOGY. [less.

something like a bishop's mitre, and hence bear the name
of the mitral valve.

The edges and apices of the vah^es are not completely
free and loose. On the contrary, a number of fine, but
strong, tendinous cords, called chorda lendinecu, connect
them with some column-like elevations of the fleshy sub-
stance of the walls of the ventricle, which are termed
papillary muscles {Y\gs. ii and I2,pp); similar column-
like elevations of the walls of the ventricles, but having
no chorda tendifiece attached to them, are called colutmicz
carncce.

It follows, from this arrangement, that the valves
oppose no obstacle to the passage of fluid from the
auricles to the ventricles ; but if any should be forced
the other way, it will at once get between the valve and
the wall of the heart, and drive the valve backwards and
upwards. Partly because they soon meet in the middle
and oppose one another's action, and partly because the
chordce tendinecc hold their edges and prevent them from
going back too far, the valves, thus forced back, give rise
to the formation of a complete transverse partition be-
tween the ventricle and the auricle, through which no fluid
can pass.

Where the aorta opens into the left ventricle and where
the pulmonary artery opens into the right ventricle
another valvular apparatus is placed, consisting in each
case of three pouch-like valves called the scmilimar
valves (Fig. ii, s.v. ; Figs. 13 and 14, Ao. P. A.), which are
similar to those of the veins. Since they are placed
on the same level and meet in the middle line, they com-
pletely stop the passage when any fluid is forced along
the artery towards the heart. On the other hand, these
valves flap back and allow any fluid to pass from the
heart into the artery, with the utmost readiness.

The action of the auriculo-ventricular valves may be
demonstrated with great ease on a sheep's heart, in which
the aorta and pulmonary artery have been tied and the
greater part of the auricles cut away, by pouring water
into the ventricles through the auriculo-ventricular aper-
ture. The tricuspid and mitral valves then usually
become closed by the upward pressure of the water
which gets behind them. Or, if the ventricles be nearly

TIIK VALVES OF THE HEART,

Fig. 12. — Left Side of the He.\rt of a Sheep (laid open).

P. V. pulmonarj' veins opening into the left auricle by four openings, as shown
by the styles ; a, a style passed from auricle into ventricle through the
auriculo-ventricular orifice ; b, a style passed into the coronary vein, vvhlch,
though it has no connexion with the left auricle, is, from its position,
necessarily cut across in thus laying open the auricle.

M. V. the two flaps of the mitral valve (drawn somewhat diagrammatically) ;
/>/, papillary muscles, belonging as before to the part of the ventricle cut
away ; c, a stj^le passed from ventricle in Ao. aorta ; .(4^^. branch of aorta
(see Fig. 8, A'o') ; P. A. pulmonarj' artery' ; .S". V.C. superior vena cava.

I, wall of ventricle cut across ; 2, wall of auricle cut away around auriculo-
ventricular orifice ; 3, other portions of auricular wall cut across ; 4, mass
of fat around base of ventricle (see Fig. 8, 2).

ELEMENTARY PHYSIOLOGY

[LtSS.

filled, the valves may be made to come together at once
by gently squeezing the ventricles. In like manner, if the
base of the aorta, or pulmonary artery, be cut out of the
heart, so as not to injure the semilunar valves, water
poured into the upper ends of the vessel will cause its
valves to close tightly, and allow nothing to flow out after
the first moment.

Thus the arrangement of the auriculo-ventricular valves
is such, that any fluid contained in the chambers of the

FA<,

m.v.t

JLAV

Fig.

-View of the Orifices of the Heart from below, the whole
OF THE Ventricles having been cut away.

R.A.V. right auriculo-ventricular orifice surrounded by the three flaps,
t.v. i, t.v. 2, t.v. 3, of the tricuspid valve ; these are stretched by weights
attached to the chorda tenditu-o'.

L A. v. left auriculo-ventricular orifice surrounded in same way by the two
flaps, 7n.z'. I, 7n.v. 2, of mitral valve ; P. A. the orifice of pulmonary artery,
the semilunar valves having met and closed together ; Ao. the orifice of the
aorta with its semilunar valves. The shaded portion, leading from R.A.V.
to P. A., represents the funnel seen in Fig. 11.

II.]

THE VALVES OF THE HEART

heart can be made to pass through the auriculo-ventricular
apertures in one direction only : that is to say, from the
auricles to the ventricles. On the other hand, the arrange-
ment of the semilunar valves is such that the fluid con-
tents of the ventricles pass easily into the aorta and
pulmonary artery, while none can be made to travel the
other way from the arterial trunks to the ventricles.

JRAV

Fig. 14. — The Orifices of the Heart seen from above, the Auricles
AND Great Vessels being cut away.

P. A. pulmonar>' artery, with its semilunar valves ; Ao. aorta, do.

R.A.V. right auriculo-ventricular orifice with the three flaps (/.Z'. i, 2, 3) of
tricuspid valve.

L.A.V. left auriculo-ventricular orifice, with m.v. i and 2, flaps of mitral
valve ; b, style passed into coronar>' vein. On the left part oi L.A. V., the
section of the auricle is carried through the auricular appendage ; hence
the toothed appearance due to the portions in relief cut across.

1 1. Like all other muscular tissues, the substance of the
heart is contractile ; but, unlike most muscles, the heart
contains within itself a something which causes its dif-
ferent parts to contract in a definite succession and at
regular intervals.

42 ELEMENTARY PHYSIOLOGY. [less.

If the heart of a hving animal be removed from the
body, it will, though in most cases for a very short time
only, go on pulsating much as it did while in the body.
And careful attention to these pulsations will show that
they consist of: — (i) A simultaneous contraction of the
walls of both auricles. (2) Immediately following this, a
simultaneous contraction of the walls of both ventricles.
(3) Then comes a pause, or state of rest ; after which the
auricles and ventricles contract again in the same order
as before, and their contractions are followed by the
same pause as before.

If the auricular contraction be represented by A", the
ventricular by V", and the pauses by — , the series of
actions will be as follows : A" V — ; A" V" — ; A" V
— ; &c. Thus, the contraction of the heart is rhytliDiical,
two short contractions of its upper and lower halves
respectively being followed by a pause of the whole,
which occupies nearly as much time as the two con-
tractions.

The state of contraction of the ventricle or auricle is
called its systole ; the state of relaxation, during which it
undergoes dilatation, its diastole.

12. Having now acquired a notion of the arrangement
of the different pipes and reserv^oirs of the circulatory
system, of the position of the valves, and of the rhyth-
mical contractions of the heart, it will be easy to com-
prehend what must happen if, when the whole apparatus
is full of blood, the first step in the pulsation of the heart
occurs and the auricles contract.

By this action each auricle tends to squeeze the fluid
which it contains out of itself in two directions — the one
towards the great veins, the other towards the ventricles ;
and the direction which the blood, as a whole, will take,
will depend upon the relative resistance offered to it in
these two directions. Towards the great veins it is
resisted by the mass of the blood contained in the veins.
Towards the ventricles, on the contrary, there is no resist-
ance Avorth mentioning, inasmuch as the valves are open,
the walls of the ventricles, in their uncontracted state,
are flaccid and easily distended, and the entire pressure
of the arterial blood is taken off by the semilunar valves,
which are necessarily closed. The return of blood into

ri.] THE ACTION OF THE VALVES. 4^

the veins is further checked by a contraction of the
great veins which immediately precedes the systole of the
auricles, and is practically continuous with it.

Therefore, when the auricles contract, little or none
of the fluid which they contain will flow back into the
veins ; all the contents or nearly so will pass into and
distend the ventricles. As the ventricles fill and begin to
resist further distension, the blood, getting behind the
auriculo-ventricular valves, will push them towards one
another, and indeed almost shut them. The auricles now
cease to contract, and immediately that their walls relax,
fresh blood flows from the great veins and slowly distends
them again.

But the moment the auricular systole is over, the
ventricular systole begins. The walls of each ventricle
contract vigorously, and the first effect of that contraction
is to complete the closure of the auriculo-ventricular
valves and so to stop all egress towards the auricle. The
pressure upon the valves becomes very considerable, and
they might even be driven upwards, if it were not for the
chordce te)idiiiecE which hold down their edges.

As the contraction continues and the capacities of the
ventricles become diminished, the points of the wall of
the heart to which the chordce tendifiece are attached ap-
proach the edges of the valves ; and thus there is a ten-
dency to allow of a slackening of these cords, which, if
it really took place, might permit the edges of the valves
to flap back and so destroy their utility. This tendency,
however, is counteracted by the cliordcs tendinecc being
connected, not directly to the walls of the heart, but to
those muscular pilla'rs, the papillary ?niiscles, which stand
out from its substance. These muscular pillars shorten
at the same time as the substance of the heart contracts ;
and thus, just so far as the contraction of the walls of the
ventricles brings the papillary muscles nearer the valves,
do they, by their own contraction, pull the chordce ten-
dinece as tight as before.

By the means which have now been described, the fluid
in the ventricle is debarred from passing back into the
auricle ; the whole force of the contraction of the ventri-
cular walls is therefore expended in overcoming the resist-
ance presented by the semilunar valves. This resistance

44 ELEMENTARY PHYSIOLOGY. [less.

is partly the result of the mere weight of the vertical
column of blood which the valves support ; but is chiefly
due to the reaction of the distended elastic walls of the
great arteries, for as we shall see, these arteries are already
so full that the blood within them is pressing on their
walls with great force.

It now becomes obvious why the ventricles have so
much more to do than the auricles, and why valves are
needed between the auricles and ventricles, while none
are wanted between the auricles and the veins.

All that the auricles have to do is to fill the ventricles,
which offer no active resistance to that process. Hence
the thinness of the walls of the auricles, and hence the
needlessness of any auriculo-venous valve, the resistance on
the side of the ventricle being so insignificant that it gives
way, at once, before the pressure of the blood in the veins.

On the other hand, the ventricles have to overcome a
great resistance in order to force fluid into elastic tubes
which are already full ; and if there were no auriculo-
ventricular valves, the fluid in the ventricles would meet
with less obstacle in pushing its way backward into the
auricles and thence into the veins, than in separating the
semilunar valves. Hence the necessity, firstly, of the
auriculo-ventricular valves ; and, secondly, of the thick-
ness and strength of the walls of the ventricles. And
since the aorta, systemic arteries, capillaries, and veins
form a system of tubes, which, from a variety of causes,
offer more resistance than do the pulmonary arteries,
capillaries, and veins, it follows that the left ventricle
needs a thicker muscular wall than the right.

Thus, at every systole of the auricles, the ventricles
are filled and the auricles emptied, the latter being slowly
refilled by the pressure of the fluid in the great veins,
which is amply sufficient to overcome the passive resist-
ance of the relaxed auricular walls. And, at every systole
of the ventricles, the arterial systems of the body and
lungs receive the contents of these ventricles, and the
emptied ventricles remain ready to be filled by the
auricles.

13. We must now consider what happens in the arteries
when the contents of the ventricles are suddenly forced into
these tubes (which, it must be recollected, are already full).

II.] THE FILLING OF THE ARTERIES. 45

If the vessels were tubes of a rigid material, like gas-
pipes, the forcible discharge of the contents of the left
ventricle into the beginning of the aorta would send a
shock, travelling with great rapidity, right along the
whole system of tubes, through the arteries into the
capillaries, through the capillaries into the veins, and
through these into the right auricle ; and just as much
blood would be driven from the end of the veins into the
right auricle as had escaped from the left ventricle into
the beginning of the aorta ; and that, at almost the
same instant of time. And the same would take place
in the pulmonary vessels between the right ventricle and
left auricle.

HowcA'er, the vessels are not rigid, but, on the contrary,
very yielding tubes ; and the great arteries, as we have
seen, have especially elastic walls. On the other hand,
the friction in the capillaries and small arteries is so
great that the blood cannot pass through them into
the veins as quickly as it escapes from the ventricle into
the aorta. Hence the contents of the ventricle, driven by
the force of the systole past the semilunar valves, are
at first lodged in the first part of the aorta, the walls
of which are stretched and distended by the extra
quantity of blood thus driven into it. But as soon as
the ventricle has emptied itself and no more blood
is driven out of it to stretch the aorta, the elastic
walls of this vessel come into play ; they strive to
go back again and make the tube as narrow as it was
before ; thus they return back to the blood the pressure
which they received from the ventricle. The effect of this
elastic recoil of the arterial walls is on the one hand to
close the semilunar valves, and so prevent the return of
blood to the heart, and, on the other hand, to distend the
next portion of the aorta, driving an extra quantity of
blood into it. And this second portion, in a similar
way, distends the next, and this again the next, and so
on, right through the whole arterial system. Thus the
impulse given by the ventricle travels like a wave along
the arteries, distending them as it goes, and ultimately
forcing the blood through the capillaries into the veins,
and so on to the heart again.

14. Several of the practical results of the working of

46 ELEMENTARY PHYSIOLOGY. [less.

the heart and arteries just described now become intel-
ligible. For example, between the fifth and sixth ribs, on
the left side, a certain movement is perceptible by the
finger and by the eye, which is known as the dealing of
the heart. It is the result of the striking of the apex of
the heart against the pericardium, and through it, on the
inner wall of the chest, at this point, at the moment of the
systole of the ventricles. Even when the heart is at rest,
the apex, in a standing position, lies close under this part
of the chest wall ; and when the systole takes place, not
only does the apex, like the rest of the ventricle, become
firm and hard, but by the peculiar movements of the heart
and great blood-vessels, is brought sharply in contact with
the chest wall at this point. It is this sudden shove of
the hardened apex which we feel and see, and which we
call the beating, or more correctly the impulse, of the
heart.

15. Secondly, if the ear be applied over the heart, cer-
tain soiDids are heard, which recur with great regularity,
at inten'als corresponding with those between ever)- two
beats. First comes a longish dull sound ; then a short
sharp sound ; then a pause ; then the long, then the sharp
sound, then another pause ; and so on. There are many
different opinions as to the cause of the first sound ; some
physiologists regard it as a muscular sound caused by the
contraction of the muscular fibres of the ventricle, while
others believe it to be due to the tension of the auriculo-
ventricular valves ; whatever be its exact cause it is given
out at the same time that the ventricles contract. The
second sound is, without doubt, caused by the sudden
closure of the semilunar valves when the ventricular systole
ends. That such is the case has been proved experi-
mentally, by hooking back the semilunar valves in a
living animal, when the second sound ceases at once.

16. Thirdly, if the finger be placed upon an arter)-,
such as that at the wrist, what is tenned \.\it pjilse will be
felt ; that is to say, the elastic arter)- dilates somewhat, at
regular intervals, which answer to the beatings of the
heart. The pulse which is felt by the finger, however,
does not correspond in time precisely with the beat of the
heart, but takes place a little after it, and the interval is
longer the greater the distance of the arter)' from the heart.

II.] THE PULSE. 47

The beat in the artery on the inner side of the ankle, for
example, is a little later than the beat of the artery in the
temple. The pulse is in fact nothing but that distension
of the arterial walls of which we spoke just now, and
which, travelling in the form of a wave from the larger to
the smaller arteries, takes longer to reach and distend the
more distant branch.

17. Fourthly, when an arter)' is cut, the outflow of the
fluid which it contains is increased hy Jt-rks, the intervals
of which correspond with the intervals of the beats of
the heart. The cause of this is plainly the same as
that of the pulse ; the force which would be employed
in distending the walls of the artery, were the latter
entire, is spent in jerking the fluid out when the arter)-
is cut.

18. Fifthly, under ordinary- circumstances, the pulse is
no longer to be detected in the capillaries, or in the veins.
This arises from several circumstances. One of them is
that the capacity of the branches of an artery is greater
than the capacity of its trunk, and the capacity of the
capillaries, as a whole, is greater than that of all the small
arteries put together. Hence, supposing the capacity of
the trunk to be 10, that of its branches 50, and that of
the capillaries into which these open 100,1 j|- jg clear that
a quantity of fluid thrown into the trunk, sufficient to
dilate it by one-tenth, and to produce a ver\- considerable
and obvious effect, could not distend each branch by more
than Jjjth, and each capillar}- by i^^th of its volume, an
effect which might be quite imperceptible.

19. But this is not all. Did the pulse merely become
indistinguishable on account of its division and dispersion
among so many capillaries, it might be felt again when
the blood is once more gathered up into a few large venous
trunks. But it is not. The pulse is definitely lost at the
capillaries. There is, under ordinar}- circumstances, no
pulse whatever in the veins, except sometimes a backward
pulse from the heart along the great venous trunks ; but
this is quite another matter.

This actual loss, or rather transformation of the pulse,

* Ten and one hundred are here taken for simplicity's sake. As a matter
of fact, the capacity of the capillaries is not only ten times, but several
hundred times greater than that of the aorta.

48 ELEMENTARY PHYSIOLOGY. [less.

is effected by means of the elasticity of the arterial walls,
in the following" manner.

In the first place it must be borne in mind that, owing
to the minute size of the capillaries and small arteries, the
amount of friction taking place in their channels when the
blood is passing through them in ver>' great ; in other
words, they offer a very great resistance to the passage of
the blood. The consequence of this is, that, in spite of
the fact that the total area of the capillaries is so much
greater than that of the aorta, the blood has a difficulty in
getting through the capillaries into the veins as fast as it
is thrown into the arteries by the heart. The whole arterial
system, therefore, becomes over-distended with blood.

Now we know by experiment that under such conditions
as these, an elastic tube has the power, if long enough
and elastic enough, to change a jerked impulse into a
continuous flow.

If a syringe (or one of the elastic bottles now so
frequently in use) be fastened to one end of a long glass
tube, and water be pumped through the tube, it will flow
from the far end in jerks, corresponding to the jerks of the
syringe. This will be the case whether the tube be quite
open at the far end, or drawn out to a fine point so as to
offer great resistance to the outflow of the water. The
glass tube is a rigid tube, and there is no elasticity to be
brought into play.

If now a long india-rubber tube be substituted for the
glass tube, it will be found to act differently, according as
the opening at the far end is wide or narrow. If it is
wide, the water flows out in jerks, nearly as distinct as
those from the glass tube. There is little resistance to
the outflow, little distension of the india-rubber tube, little
elasticity brought into play. If, however, the opening be
narrowed, as by fastening to it a stopcock or a glass tube
drawn to a point, or if a piece of sponge be thrust into
the end of the tube — if, in fact, in any way resistance be
offered to the outflow of the water, the tube becomes
distended, its elasticity is brought into play, and the
water flows out from the end, not in jerks but in a stream,
which is more and more completely continuous the longer
and more elastic the tube.

Substitute for the syringe the heart, for the stopcock or

II. J THE PULSE. 49

sponge the capillaries and small arteries, for the india-
rubber tube the whole arterial system, and you have
exactly the same result in the living body. Through the
action of the elastic arterial walls the separate jets from
the heart are blended into one continuous stream. The
whole force of each blow of the heart is not at once spent
in driving a quantity of blood through the capillaries ; a
part only is thus spent, the rest goes to distend the elastic
arteries. But during the interval between that beat and
the next the distended arteries are narrowing again, by
virtue of their elasticity, and so are pressing the blood on
into the capillaries with as much force as they were
themselves distended by the heart. Then comes another
beat, and the same process is repeated. At each stroke
the elastic arteries shelter the capillaries from part of the
sudden blow, and then quietly and steadily pass on that
part of the blow to the capillaries during the interval
between the strokes.

The larger the amount of elastic arterial wall thus
brought into play, i.e. the greater the distance from the
heart, the greater is the fraction of each heart's stroke
which is thus converted into a steady elastic pressure
between the beats. Thus the pulse becomes less and
less marked the farther you go from the heart ; any given
length of the arterial system, so to speak, being sheltered
by the lengths between it and the heart.

Every inch of the arterial system may, in fact, be con-
sidered as converting a small fraction of the heart's jerk
into a steady pressure, and when all these fractions are
summed up together in the total length of the arterial
system n.o trace of the jerk is left.

As the immediate, sudden effect of each systole becomes
diminished in the smaller vessels by the causes above
mentioned, that of this constant pressure becomes more
obvious, and gives rise to a steady passage of the fluid
from the arteries towards the veins. In this way, in fact,
the arteries perform the same functions as the air-reservoir
of a fire-engine, which converts the jerking impulse
given by the pumps into the steady flow of the delivery
hose.

20. Such is the general result of the mechanical condi-
tions of the organs of the circulation combined with the

i£ E

50 ELEMENTARY PHYSIOLOGY. [less.

rhythmical activity of the heart. This activity drives the
fluid contained in these organs out of the heart into the
arteries, thence to the capillaries, and from them through
the veins back to the heart. And in the course of these
operations it gives rise, incidentally, to the beating of the
heart, the sounds of the heart, and the pulse.

It has been found, by experiment, that in the horse it
takes about half a minute for any substance, as for in-
stance a chemical body, whose presence in the blood can
easily be recognized, to complete the circuit, e.i: gr. to pass
from the jugular vein down through the right side of the
heart, the lungs, the left side of the heart, up through the
arteries of the head and neck, and so back to the jugular
vein.

By far the greater portion of this half minute is taken
up by the passage through the capillaries, where the
blood moves, it is estimated, at the rate only of about one
and a half inches in a /ninute, whereas through the carotid
artery of a dog it flies along at the rate of about ten
inches in a second. Of course to complete the circuit
of the circulation, a blood-corpuscle need not have to go
through so much as half of an inch of capillaries in either
the lungs or any of the tissues of the body.

Inasmuch as the force which drives the blood on is
(putting the other comparatively slight helps on one
side) the beat of the heart and that alone, however much
it may be modified, as we have seen, in character, it is
obvious that the velocity with which the blood moves
must be greatest in the aorta and diminish towards the
capillaries.

For with each branching of the arteries the total area
of the arterial system is increased, the total width of the
capillary tubes if they were all put together side by side
being very much greater than that of the aorta. Hence
the blood, or a corpuscle, for instance, of the blood being
driven by the same force, viz. the heart's beat, over the
whole body, must pass much more rapidly through the
aorta than through the capillary system or any part of
that system.

It is not that the greater friction in any capillary-
causes the blood to flow more slowly there and there
only. The resistance caused by the friction in the

II.] PORTAL CIRCULATION. 51

capillaries is thrown back upon the aorta, which
indeed feels the resistance of the whole vascular
system ; and it is this total resistance which has to
be overcome by the heart before the blood can move
on at all.

The blood driven everywhere by the same force simply
moves more and more slowly as it passes into wider and
wider channels. When it is in the capillaries it is slowest ;
after escaping from the capillaries, as the veins unite into
larger and larger trunks, and hence as the total venous
area is getting less and less, the blood moves again faster
and faster for just the same reason that in the arteries it
moved slower and slower.

A very similar case is that of a river widening out in a
plain into a lake and then contracting into a narrow stream
again. The water is driven by one force throughout (that
of gravity). The current is much slower in the lake than
in the narrower river either before or behind.

21. It is now necessary to trace the exact course of
the circulation as a w^hole. And we may conveniently
commence with the portion of the blood contained at any
moment in the right auricle. The contraction of the right
auricle drives that fluid into the right ventricle ; the ven-
tricle then contracts and forces it into the pulmonary
artery ; from hence it passes into the capillaries of the
lungs. Leaving these, it returns by the four pulmonary
veins to the left auricle ; and the contraction of the left
auricle drives it into the left ventricle.

The systole of the left ventricle forces the blood into
the aorta. The branches of the aorta convey it into all
parts of the body except the kmgs ; and from the capil-
laries of all these parts, except from those of the stomach
intestines and certain other viscera in the abdomen,
it is conveyed, by vessels which gradually unite into
larger and larger trunks, into either the superior or
the inferior vena cava^ which carry it to the right auricle
once more.

But the blood brought to the capillaries of the stomach
and intestines, spleen and pancreas, is gathered into veins
which unite into a single trunk — the vena porice. The
vena portse distributes its blood to the liver, mingling
with that supplied to the capillaries of the same organ by

E 2

$i KLEMENTARV rilYSIOLOGV. [less.

the hepatic artery. From these capiharies it is conveyed
by small veins, which unite into a large trunk — the
hepatic veiii^ which opens into the inferior vena cava.
The flow of the blood from the abdominal viscera,
through the liver, to the hepatic vein, is called the portal
circulation.

The heart itself is supplied with blood by the two
coronary arteries which spring from the root of the aorta
just above two of the semilunar valves. The blood from
the capillaries of the heart is carried back by the coronarj'
vein, not to either vena cava, but to the right auricle. The
opening of the coronary vein is protected by a valve, so
as to prevent the right auricle from driving the venous
blood which it contains back into the vessels of the
heart.

22. Thus, the shortest possible course which an)- particle
of the blood can take in order to pass from one side of
the heart to the other, is to leave the aorta by one of the
coronary arteries, and return to the right auricle by the
coronar}- vein. And in order to pass through the greatest
possible number of capillaries ■SiW^ return to the point from
which it started, a particle of blood must leave the heart
by the aorta and traverse the arteries which supply the
alimentary canal, spleen and pancreas. It then enters
I stly, the capillaries of these organs ; 2ndly, the capillaries
of the liver ; and, 3rdly, after passing through the right
side of the heart, the capillaries of the lungs, from which
it returns to the left side and eventually to the aorta.

Furthermore, from what has been said respectidig the
lymphatic system, it follows that any particle of matter
which enters a lacteal of the intestine, will reach the right
auricle by the superior cava, after passing through the
lymph capillaries and channels of sundry lymphatic
glands ; while anything which enters the adjacent blood
capillary in the wall of the intestine will reach the right
auricle by the inferior cava, after passing through the
blood capillaries of the liver.

23. It has been shown above (§ 2) that the small
arteries may be directly affected by the nervous system,
which controls the state of contraction of their muscular
walls, and so regulates their calibre. The effect of this
power of the nervous system is to give it a certain

II.] VASO-MOTOR ACTION. 53

control over the circulation in particular spots, and to
produce such a state of affairs that, although the force of
the heart and the general condition of the vessels remain
the same, the state of the circulation may be very dif-
ferent in different localities.

Bliishiii'g is a purely local modification of the circu-
lation of this kind, and it will be instructive to consider
how a blush is brought about. An emotion, sometimes
pleasurable, sometimas painful, takes possession of the
mind ; thereupon a hot flush is felt, the skin grows red,
and according to the intensity of the emotion these
changes are confined to the cheeks only, or extend to the
" roots of the hair," or " all over."

What is the cause of these changes? The blood is a
red and a hot fluid ; the skin reddens and grows hot,
because its vessels contain an increased cjuantity of this
red and hot fluid ; and its vessels contain more, because
the small arteries suddenly dilate, the natural moderate
contraction of their muscles being superseded by a state
of relaxation ; and this relaxation comes on because the
action of the nervous system which previously kept the
muscles in a state of moderate contraction is, for the
time, suspended.

On the other hand, in many people, extreme terror
causes the skin to grow cold, and the face to appear pale
and pinched. Under these circumstances, in fact, the
supply of blood to the skin is greatly diminished, in con-
sec[uence of an increased contraction of the muscles of the
small arteries whereby these become unduly narrowed
or constricted, and thus allow only a small cjuantity of blood
to pass through them ; and this increased contraction of the
muscular coats of the arteries is brought about by the
increased action of the nervous system.

24. That this is the real state of the case may be proved
experimentally upon rabbits. These animals may be made
to blush artificially. If, in a rabbit, the sympathetic nerve
which sends branches to the vessels of the head is cut,
the ear of the rabbit, which is covered by so delicate an
integument that the changes in its vessels can be readily
perceived, at once blushes. That is to say, the vessels
dilate, fill with blood, and the ear becomes red and hot.
The reason of this is^ that when the sympathetic is cut,

54 ELEMENTARY PHYSIOLOGY. [lf.ss.

the nervous stimulus which is ordinarily sent along its
branches is interrupted, and the muscles of the small
vessels, which were slightly contracted, become altogether
relaxed.

And now it is quite possible to produce pallor and cold
in the rabbit's ear. To do this it is only necessary to
irritate the cut end of the sympathetic which remains
connected with the vessels. The nerve then becomes
excited, so that the muscular fibres of the vessels are
thrown into a violent state of contraction, which di-
minishes their calibre so much that the blood can hardly
make its way through them. Consequently, the ear
becomes pale and cold.

25. The nerves which thus regulate the calibre of the
small arteries by acting on their muscular coats are
called vaso-7notor nerves ; and through them the nervous
system is able to exert a local control over the circu-
lation in any part or organ, the importance of which
is very great. Thus, when an organ becomes active,
it is of advantage that it should be more richly supplied
with blood than when it is at rest. Accordingly we find
that when a muscle contracts, or when a salivary gland
secretes saliva, or when the stomach is preparing to
digest food, in each case the small arteries of the
muscle, salivary gland or stomach, dilate and so flush the
part with blood. The organ in fact blushes ; and this
inner unseen blushing is, like the ordinary blushing
described above, brought about by vaso-motor nerves.
We shall see later on that the temperature of the body is
largely regulated by the supply of blood sent to the skin
to be cooled, and this supply is in turn regulated by the
vaso-motor nervous system. Indeed everywhere all over
the body, the nervous system by its vaso-motor nerves is
continually supervising and regulating the supply of blood,
sending now more now less blood, to this or that part ;
and many diseases, such as those when exposure to cold
causes congestion or inflammation, are due to, or at least
associated with, a disorder or failure of this vaso-motor
activity.

26. Is the heart, in like manner, under the control of
the central nervous system .''

As we all know, it is not under the direct influence of

ir.] NERVES OF THE HEART. 55

the will, but every one is no less familiar with the fact
that the actions of the heart are wonderfully affected by
. all forms of emotion. Men and women often faint, and
have sometimes been killed by sudden and violent joy or
sorrow ; and when they faint or die in this way, they do
so because the perturbation of the brain gives rise to a
something which arrests the heart as dead as you stop a
stop-watch with a spring. On the other hand, other emo-
tions cause that extreme rapidity and violence of action
which we call palpitation.

Now the heart is well supplied with ner\-es. There are
many small ganglia, or masses of nerve cells lodged in
the substance of the heart, more especially in the auricles,
and nerves spread from these ganglia over the walls, both
of the auricles and ventricles. Moreover, several nerves
reach the heart from the outside. Of these the most
important perhaps are branches of a remarkable nerve
which starts from the brain, and supplies not only the
heart, but the lungs, alimentary canal, and other parts,
and which is called the p7tejcmooast?'ic, or from its wan-
dering course, the vagus. Other nerves reaching the
heart seem to come from the sympathetic, but probably
many of these may be traced back through the sympathetic
to the spinal cord. There is every reason to believe that
the regular rhythmical succession of the ordinary contrac-
tions of the heart depends in some way upon the ganglia
lodged in its substance. At any rate, it is certain that
these movements do not depend on any nerves outside
the heart, since they go on even when the heart is re-
moved from the body.

On the other hand the influence which arrests the
heart's action, as in fainting, comes to the heart from
without, and is carried to the heart by the pneumogastric.
This may be demonstrated in animals, such as frogs, with
great ease.

27. If a frog be pithed, or its brain destroyed, so as to
obliterate all sensibility, the animal will continue to live,
and its circulation will go on perfectly well for an inde-
finite period. The body may be laid open without
causing pain or other disturbance, and then the heart
will be observed beating with great regularity. It is
^ possible to make the heart move a long index backwards

ELKMEXTARV PHYSIOLOGY

[less.

and forwards ; and if frog and index are covered with a
glass shade, the air under which is kept moist, the index
will vibrate with great steadiness for a couple of days.

Fig. 15.— Portion of the web of a frog's foot seen under a low magnif\-ing
power, the blood-vessels only being represented, except in the comer of the
field, where in the portion marked ofif the pigment spots are also drawn.
a. small arteries ; v. small veins : the minute tubes joining the arteries of the
veins are the capillaries. The arrows denote the direction of the circula-
tion. The larger arter>- running straight up in the middle line breaks up
into capillaries at points higher up than can be shown in the drawing.

II.] EVIDENXES OF THE CIRCULATION'. 57

It is easy lo adjust to the frog thus prepared a contri-
vance by which electrical shocks may be sent through
the pneumogastric nenes, so as to irritate them. The
moment this is done the index stops dead, and the heart
will be found quiescent, with relaxed and distended walls.
After a little time the influence of the pneumogastric
passes off, the heart recommences its work as vigorously
as before, and the index vibrates through the same arc as
formerly. With careful management, this experiment
may be repeated very many times ; and after ever)- arrest
by the irritation of the pneumogastric, the heart resumes
its work. When a person faints from a sudden emotion,
a similar influence, started in the brain, descends along
the pneumogastric, and similarly srops for a while the
beating of the heart.

The exact manner in which palpitation is brought about
does not seem so clear ; in such cases an influence of some
kind probably reaches the heart along ner\-es which for a
part of their course run along with the sympathetic nerves ;
but this subject requires further investigation.

28. The evidence that the blood circulates in man, al-
though perfectly conclusive, is almost all indirect. The
most important points in the e\idence are as follows : —

In the first place, the disposition and structure of the
organs of circulation, and more especially the arrange-
ment of the various valves, will not, as was shown by
Hars'ey, permit the blood to flow in any other direction
than in the one described above. Moreover, we can
easily with a syringe inject a fluid from the vena cava, for
instance, through the right side of the heart, the lungs,
the left side of the heart, the arteries, and capillaries, back
to the vena cava : but not the other way. In the second
place, we know that in the living body the blood is con-
tinually flowing in the arteries towards the capillaries,
because when an arter}- is tied, in a living body, it swells
up and pulsates on the side of the ligature nearest the
heart, whereas on the other side it becomes empt}-, and
the tissues supplied by the arter\- become pale from the
want of a supply of blood to their capillaries. And when
we cut an arter>- the blood is pumped out in jerks from
the cut end nearest the heart, whereas little or no blood
comes from the other end. When, however, we tie a vein

ELEMENTARY PHYSIOLOGY

[lf.ss.

Fig. i6. — Ver>' small portion of Fig. 15 ver>- highly magnified.

A. walls of capillaries;^, tissue of web lying between the capillaries;
C. cells of epidermis covering web (these are only shown in the right-hand

II.] EVIDENCES OF THE CIRCULATION. 59

the state of things is reversed, the swelHng taking place
on the side farthest from the heart, &c. Sec, showing that
in the veins the blood flows from the capillaries to the
heart.

But certain of the lower animals, the whole, or parts,
of the body of which are transparent, readily afford direct
proof of the circulation ; in these the blood may be seen
rushing from the arteries into the capillaries, and from the
capillaries into the veins, so long as the animal is alive
and its heart is at work. The animal in which the circu-
lation can be most conveniently observed is the frog.
The web between its toes is very transparent, and the
particles suspended in its blood are so large that they can
be readily seen as they slip swiftly along with the stream
of blood, when the toes are fastened out, and the inter-
vening web is examined under even a low magnifying
power (Figs. 15 and 16).

and lower part of the field ; in the other parts of the field the focus of the
microscope lies below the epidermis) ; £). nuclei of these epidermic cells ; £.
pigment cells contracted, not partially expanded as in Fig. 15 ; /^. red
blood-corpuscle (oval in the frog) passing along capillary- — nucleus not
visible ; G. another corpuscle squeezing its way through a capillar^', the
canal of which is smaller than its own transverse diameter ; //. another
bending as it slides round a comer ; A', corpuscle in capillary- seen through
the epidermis ; /. white blood-corpuscle. "

6o ELEMENTARY I'llVSIOLOGY. [less.

LESSON IIL

THE BLOOD AND THE LYMPH.

1. In order to become properly acquainted with the
characters of the blood it is necessary to examine it with
a microscope magnifying at least three or four hundred
diameters. Provided with this instrument, a hand lens,
and some slips of thick and thin glass, the student will be
enabled to follow the present lesson.

The most convenient mode of obtaining small quantities
of blood for examination is to twist a piece of string,
pretty tightly, round the middle of the last joint of the
middle, or ring finger, of the left hand. The end of the
finger will immediately swell a little, and become darker
coloured, in consequence of the obstruction to the return
of the blood in the veins caused by the ligature. When
in this condition, if it be slightly pricked with a sharp
clean needle (an operation which causes hardly any pain),
a good-sized drop of blood will at once exude. Let it be
deposited on one of the slips of thick glass, and covered
lightly and gently with a piece of the thin glass, so as to
spread it out evenly into a thin layer. Let a second slide
receive another drop, and, to keep it from drying, let it be
put under an inverted watch-glass or wine-glass, with a
bit of wet blotting-paper inside. Let a third drop be
dealt with in the same way, a few granules of common
salt being first added to the drop.

2. To the naked eye the layer of blood upon the first
slide will appear of a pale reddish colour, and quite clear
and homogeneous. But on viewing it with even a pocket
lens its apparent homogeneity will disappear, and it will

III.] RED CORPUSCLES OF THE BLOOD., 6t

look like a mixture of excessively fine yellowish-red par-
ticles, like sand, or dust, with a watery, almost colourless,
fluid. Immediately after the blood is drawn, the particles
will appear to be scattered very evenly through the fluid,
but by degrees they aggregate into minute patches, and
the layer of blood becomes more or less spotty.

The " particles " are what are termed the cor'piiscles of
the blood ; the nearly colourless fluid in which they are
suspended is the plasma.

The second slide may now be examined. The drop of
blood will be unaltered in form, and may perhaps seem to
have undergone no change. But if the slide be inclined,
it will be found that the drop no longer flows ; and, indeed,
the slide may be inverted without the disturbance of the
drop, which has become solidified, and may be removed,
with the point of a penknife, as a gelatinous mass. The
mass is quite soft and moist, so that this setting, or coagu-
lation, of a drop of blood is something very different from
its drying.

On the third slide, this process of coagulation will be
found not to have taken place, the blood remaining as
fluid as it was when it left the body. The salt therefore,
has prevented the coagulation of the blood. Thus this
very simple investigation teaches that blood is composed
of a nearly colourless plasma, in which many coloured
corpuscles are suspended ; that it has a remarkable power
of coagulating ; and that this coagulation may be pre-
vented by artificial means, such as the addition of salt.

3. If, instead of using the hand lens, the drop of blood
on the first slide be placed under the microscope, the par-
ticles, or corpuscles, of the blood will be found to be
bodies with very definite characters, and of two kinds,
called respectively the red corpuscles and the colourless
corpuscles. The former are much more numerous than
the latter, and have a yellowish-red tinge ; when one of
these corpuscles is seen, under a high power of the
microscope, lying by itself, it seems to be hardly more
than faintly yellow in colour, but when several are seen
lying one on the other, the redness becomes obvious. The
latter, somewhat larger than the red corpuscles, are, as
their name implies, pale and devoid of coloration.

4. The corpuscles difter also in other and more

ELEMENTARY PHYSIOLOGY.

[less.

important respects. The red corpuscles (Fig. 17) are
flattened circular disks, on an average y/.Toth of an inch
in diameter, and having about one-fourth of that thickness.
It follows that rather more than 10,000,000 of them will

^^^

Fig. 17. — Red and White Corpuscles of the Blood Magnified.

A. Moderately magnified. The red corpuscles are seen lying in rouleaux;
at a and a are seen two white corpuscles. _

B. Red corpuscles much more highly magnified, seen in face ; C. ditto, seen
in profile ; D. ditto, in rouleaux, rather more highly magnified ; E. a red
corpuscle swollen into a sphere by imbibition of water.

F. A white corpuscle magnified same as B. ; G. ditto, throwing out some
blunt processes ; K. ditto, treated with acetic acid, and showing nucleus
magnified same as D.

H. Red corpuscles puckered or crenate all over.

/. Ditto, at the edge only.

lie on a space one inch square, and that the volume of
each corpuscle does not exceed T^TTyrcVyxnTiruth of a
cubic inch.

III.] RED CORrUSCLES. 63

The broad laces of the disks arc not llat, but somewhat
concave, as if they were pushed in towards one another.
Hence the corpuscle is thinner in the middle than at the
edges, and when viewed under the microscope, by trans-
mitted light, looks clear in the middle and darker at
the edges, or dark in the middle and clear at the edges,
according as it is or is not in focus. When, on the other
hand, the disks roll over and present their edges to the
eye, they look like rods. All these varieties of appear-
ance may be made intelligible by turning a round biscuit
or muffin, bodies more or less similar in shape to the red
corpuscles, in various ways before the eye.

The red corpuscles are very soft, flexible, and elastic
bodies, so that they readily squeeze through apertures
and passages narrower than their own diameters, and im-
mediately resume their proper shapes (Fig. 16, G.H.).
Examined under even a high power the red corpuscle
presents no very obvious structure ; when however blood
is frozen and thawed one or more times, or when it is
treated in certain other ways, the colouring matter which
gave each corpuscle its yellow or yellowish red tinge is
dissolved out and passes into the plasma, and all that is
left of the corpuscle is a colourless framework appearing
often under the microscope as a pale, hardly visible, ring.
Each corpuscle in fact consists of a sort of spongy
colourless framework composed of the kind of material
known 2.%proteid (see lesson I. § 4) and of a peculiar colour-
ing matter, which, in the natural condition, is intimately
connected with this framework but may, by appropriate
means be removed from it. This colouring matter, which
is of a highly complex nature, is called haemoglobin and
may, by proper chemical treatment be resolved into a
reddish brown substance containing iron, called haematin,
and a colourless proteid substance.

Each corpuscle therefore is not to be considered as a
bag or sack with a definite skin or envelope containing
fluid, but rather as a sort of spongy semi-solid or semi-fluid
mass, like a disc of soft jelly ; and as such is capable of
imbibing water and swelling up, or giving out water and
shrinking according to the density of the fluid in which it
may be placed. Thus, if the plasma of blood be made
denser by dissolving saline substances, or sugar, in it,

64 KLEMENTARV PHYSIOLOGY. [less.

water is drawn from the substance of the corpuscle to the
dense plasma, and the corpuscle becomes still more
flattened and very often much wrinkled. On the other
hand, if the plasma be diluted with water, the latter forces
itself into and dilutes the substance of the corpuscle,
causing the latter to swell out, and even become spherical ;
and, by adding dense and weak solutions alternately, the
corpuscles may be made to become successively spheroidal
and discoidal. Exposure to carbonic acid gas seems to
cause the corpuscles to swell out ; oxygen gas, on the
contrary, appears to flatten them.

5. The colourless corpuscles (Fig. ij, a a, F. G. K.) are
larger than the red corpuscles, their average diameter
being ^-gVoth of an inch. They are further seen, at a glance,
to differ from the red corpuscles by the extreme irregularity
of their form, and by their greater stickiness or adhesive-
ness, shown by their tendency to attach themselves to the
glass slide, while the red corpuscles float about and tumble
freely over one another.

A still more remarkable feature of the colourless
corpuscles than the irregularity of their form is the
unceasing variation of shape which they exhibit so long as
they are alive. The form of a red corpuscle is changed
only by influences from v.ithout, such as pressure, or the
like ; that of the colourless corpuscle is undergoing
constant alteration, as the result of changes taking place
in its own substance. To see these changes well, a
microscope with a magnifying power of five or six hundred
diameters is requisite ; and, even then, they are so gradual
that the best way to ascertain their existence is to make a
drawing of a given colourless corpuscle at intervals of a
minute or two. This is what has been done with the
corpuscle represented in Fig. 18, in which a represents the
fonn of the corpuscle when first observed ; l>, its form a
minute afterwards ; c, that at the end of the second ; d,
that at the end of the third ; and e, that at the end of the
fifth minute.

Careful watching of a colourless corpuscle, in fact,
shows that every part of its surface is constantly changing
— undergoing active contraction or being passively dilated
by the contraction of other parts. It exhibits contractility
in its lowest and most primitive form.

III.] COLOURLESS CORPUSCLES. . 65

6. While they are thus Hving and active, a complete
knowledge of the stnicture of the colourless corpuscles
cannot be arrived at. Each corpuscle seems to be formed
simply of a mass of the finely or coarsely granular
substance called protoplasm in which no distinction of
parts can be seen. This is especially the case when the
corpuscle is at rest and assumes a spheroidal shape.
Sometimes, however, the corpuscle in the course of the
movements just described, spreads itself out into a ver)-
thin flat film ; and when that is the case there may
be seen in its interior a rounded body, differing in
appearance from the rest of the body of corpuscle.
Again when a drop of blood is diluted with water, still
better with ver}- dilute acetic acid, the spong}- protoplasm
of the w hite corpuscles swells up and becomes transparent,

€t 1, c d e

Fig. i3. — Successive Forms assumed by Colourless Corpuscles of
Human Blood. (Magnified about 6co diameters.)

The )nter\als between the forms a,b,c.d. was a minute ; between d and e two
minutes ; so that the whole series of changes from a to «? took five minutes.

many of the granules becoming dissolved, and in this
case' the same rounded body becomes visible. This
internal rounded body, which diflfers in nature from the
rest of the substance of the corpuscles is called the micleus
(Fig. 17, IC J ; and when the blood is treated under the
microscope, with various staining fluids, such as solutions
of carmine or logsvood, the nucleus generally stains more
deeply than the rest of the corpuscle.

The colourless corpuscle, with its nucleus, is what is
called a nucleated cell. It will be obsened that it lives
in a free state in the plasma of the blood, and that it
exhibits an independent contractilit)-. In fact, except that
it is dependent for the conditions of its existence upon the
plasma, it might be compared to one of those simple
organisms which are met with in stagnant water, and are
called Am(£bce.

66 ELEMENTARY PIIYSIOLOGV. [less.

7. But while the colourless corpuscles are thus nucleated
cells, the red corpuscles have no such nucleus ; and this is
true not only of human blood but of the blood of all
mammals, i.e. of all those animals which suckle their
young ; in all these the red corpuscle has no nucleus. In
the case of birds, reptiles and fishes, however, the red
corpuscles as well as the colourless are nucleated ; and in
the embryos ^ even of mammals the red corpuscles are at
first nucleated.

The exact number of both red and colourless corpuscles
present in the blood varies a good deal from time to time ;
and there is reason to think that both kinds of corpuscles
are continually being destroyed or made use of, their place
being supplied by new corpuscles. P\irther, there is
reason to think that colourless corpuscles are formed, in
part at least, in the lymphatic glands, from whence they
pass through the lymphatic vessels into the blood, and that
the red corpuscles are formed, probably in particular parts
of the body, by the formation of haemoglobin in cells
which previously contained no such colouring matter.
But whether the cells which give rise to red corpuscles
are ordinary white corpuscles or a particular kind of
cell, and how it is that the mammalian red corpuscle
comes to have no nucleus, are questions, not as yet
definitely decided.

8. As the blood dies, its several constituents, which
have now been described, undergo marked changes.

The colourless corpuscles lose their contractility, but
otherwise undergo little alteration. They tend to cohere
neither with one another, nor with the red corpuscles, but
adhere to the glass plate on which they are placed.

It is quite otherwise with the 7'ed corpuscles^ which at
first, as has been said, float about and roll, or slide, over
each other quite freely. After a short time (the length of
which varies in different persons, but usually amounts to
two or three minutes), they seem, as it were, to become
sticky, and tend to cohere ; and this tendency increases
until, at length, the great majority of them become applied
face to face, so as to form long series, like rolls of coin.
The end of one roll cohering with the sides of another,

* An embryo is the rudimentary unborn young of any creature,
1

iir.l BLOOD-CRYSTALS. 67

a network of various degrees of closeness is produced
(Fig. 17, J.).

'Ihe corpuscles remain thus coherent for a certain
length of time, but eventually separate and float freely
again. The addition of a little water, or dilute acids or
sahne solutions, will at once cause the rolls to break up.

It is from this running of the corpuscles together into
patches of network that the change noted above in the
appearances of the layer of blood, viewed with a lens,
arises. So long as the corpuscles are separate, the sandy
appearance lasts ; but when they run together, the layer
appears patchy or spotted.

The red corpuscles rarely, if ever, all run together into
rolls, some always remaining free in the meshes of the
net. In contact with air, or if subjected to pressure, many
of the red corpuscles become covered with little knobs, so
as to look like minute mulberries — an appearance which
has been mistaken for a breaking up, or spontaneous
division, of the corpuscles (Fig. 17, //.//.).

9. There is a still more remarkable change which the
red blood-corpuscles occasionally undergo. Under certain
circumstances, the peculiar red substance which gives
them their colour, and indeed forms the chief part of
their mass, and which has been called hcEmoglobin (§ 4),
separates in a crystalline form. In man, these crystals
have the shape of prisms ; in difterent animals they take
different forms. Human blood crystallizes with difficulty,
but that of the guinea-pig, rat, or dog much more easily.

If a little rat's or dog's blood, from which the fibrin has
been removed, be shaken up with a little ether, and
allowed to stand in the cold for some hours, a sediment
will frequently form at the bottom ; and this sediment
when examined with the microscope, will be found to
consist chiefly of long narrow blood-crystals.

10. When the layer of blood has been drawn ten or
fifteen minutes, the plasma will be seen to be no longer
clear. It then exhibits multitudes of extremely delicate
filaments of a substance called Fibrin, which have been
formed in it, and which traverse it in all directions,
uniting with one another and with the corpuscles, and
binding the whole into a semi-solid mass.

It is this formation of fibrin which is the cause of

68 ELEMENTARY rilVSIOLOGV [less.

the apparent solidification, or coagulation, of the drop
upon the second slide ; but the phenomena of coagula-
tion, which are of very- great importance, cannot be
properh' understood until the behaviour of the blood,
when drawn in larger quantity than a drop, has been
studied.

11. When, by the ordinar)- process of opening a vein
with a lancet, a quantity of blood is collected into a basin,
it is at first perfectly fluid : but in a very few minutes it
becomes, through coagulation, a jelly-like mass, so solid
that the basin may be turned upside down without any of
the blood being spilt. At first the clot is a uniform red
jelly, but very soon drops of a clear yellowish watery-
looking fluid make their appearance on the surface of the
clot, and on the sides of the basin. These drops increase
in number, and run together, and after a while it has
become apparent that the originally unifonn jelly has
separated into two ver)- different constituents— the one a
clear, yellowish liquid ; the other a red, semi-solid mass,
which lies in the liquid, and at the surface is sometimes
paler in colour and firmer than in its deeper part.

The liquid is called the sei'um ; the semi-solid mass the
clot, or o'assanicntuni. Now the clot obviously contains
the corpuscles of the blood, bound together by some
other substance ; and this last, if a small part of the clot
be examined microscopically, will be found to be that
fibrous-looking matter, /■<^;v>/, which has been seen fomi-
ing in the thin layer of blood. Thus the clot is equiva-
lent to the corpuscles plus the fibrin of the plasma, while
the serum is the plasma nimus the fibrinous elements
which it contained.

12. The corpuscles of the blood are slightly heavier
than the plasma, and therefore, when the blood is drawn,
they sink very slowly towards the bottom. Hence the
upper part of the clot is apt to contain fewer corpuscles,
and to be lighter in colour, than the lower part— there
being fewer corpuscles left in the upper layer of plasma
for the fibrin to catch when it sets. When the blood clots
slowly, the corpuscles have so much time to sink that
the upper stratum of plasma becomes quite free from
red corpuscles before the fibrin forms in it ; and, conse-
quently, the uppermost layer of the clot is nearly white :

III.] COAGULATION OF BLOOD. . 69

it then receives the name of the buffy coat. This is well
seen in the blood of the horse. Sometimes the rapid
sinking of the corpuscles and hence the appearance of
the bufty coat appears to be due to some conditions
of the blood causing- the corpuscles to run together much
more closely and in denser masses than usual, whereby
they more readily overcome the resistance of the plasma
to their falling, just as feathers stuck together in masses
fall much more rapidly through the air than the same
feathers when loose.

After the clot is formed, the fibrin shrinks and squeezes
out much of the serum contained within its meshes ; and,
other things being equal, it contracts the more the fewer
corpuscles there are in the way of its shrinking. Hence,
when the buffy coat is formed, it usualjy contracts so much
as to give the clot a cup-like upper surface.

Thus the buffy coat is fibrin naturally separated irom
the red corpuscles ; the same separation may be effected,
artificially, by whipping the blood with twigs as soon as
it is drawn, until its coagulation is complete. Under
these circumstances the fibrin will collect upon the twigs,
and a red fluid will be left behind, consisting of the serum
plKS the red corpuscles, and many of the colourless ones.

13. The coagulation of the blood is hastened, retarded,
or temporarily prevented by many circumstances.

{cj) TcDiperatiire. — A high temperature accelerates the
coagulation of the blood ; a low one retards it \ ery
greatly ; so much so that blood kept at a temperature
close to freezing point, may remain fluid for a very long
time indeed.

ib) The addition of saline matters to the blood. — Many
saline substances, and more especially sulphate of soda
and common salt, dissolved in the blood in sufficient
c[uantity, prevent its coagulation ; but coagulation sets in
when water is added, so as to dilute the saline solution.

(^) Contact luitJi living or not living matter. — Contact
with not living matter promotes the coagulation of the
blood. Thus, blood drawn into a basin begins to coagu-
late first where it is in contact with the sides of the
basin ; and a wire introduced into a living vein will
become coated with fibrin, although perfectly fluid blood
surrounds it.

70 KLEMEXTARY rilVSlULOGY. [less.

On the otlier hand, direct contact with living matter
retards, or altogether prevents, the coagulation of the
blood. Thus blood remains fluid for a very long time in
a portion of a vein which is tied at each end. The heart
of a turtle remains alive for a lengthened period (many
hours or even days) after it is extracted from the body ;
and, so long as it remains alive, the blood contained in it
will not coagulate, though, if a portion of the same
blood be removed from the heart, it will coagulate in a
few minutes. Blood taken from the body of the turtle,
and kept from coagulating by cold for some time, may be
poured into the separated, but still living, heart, and then
will not coagulate.

Freshly deposited fibrin acts somewhat like living
matter, coagulable blood remaining fluid for a long time
in tubes coated with such fibrin.

14. The coagulation of the blood is an altogether
physico-chemical process, dependent upon the properties
of certain of the constituents of the plasma, apart from
the vitality of that fluid. This is proved by the fact that
if common table salt be gradually added to freshly-drawn
blood which has not yet coagulated, or to blood plasma
which has been kept from coagulating by cold, a white,
flocculent, somewhat viscid substance is thrown down or
precipitated as soon as sufficient salt has been added.
The substance so thrown down may be separated by filtra-
tion, and purified by washing with a concentrated solution
of salt, in which it is insoluble. It is not fibrin, for
whereas fibrin is characteristically insoluble, this substance
is readily soluble in distilled water, giving a clear limpid
solution.' But this solution does not long remain so ;
unless special precautions, such as exposing to cold, &c.,
be taken, it soon becomes viscid, then turns into a jelly,
and at last forms an unmistakable clot of true fibrin. The
substance in question is therefore an antecedent of fibrin,
which, by some changes or other, is converted into fibrin ;
that is to say, the coagulation of blood is due to the con-
version of this soluble antecedent of fibrin into insoluble
fibrin.

The exact nature of the changes involved in this con-
version have not even yet been thoroughly worked out ;
but the following facts deserve attention; — The peri-

III.] COAGULATION OF ULOOD. 71

cardium and other serous cavities in the body contain a
clear fluid, which may be briefly described as consisting of
the elements of the blood without the red blood-corpuscles.
This fluid sometimes coagulates spontaneously, as the
blood plasma would do, but ^•cry often shows no disposi-
tion to spontaneous coagulation. When the latter is the
case, the fluid may nevertheless be made to coagulate,
and yield a true fibrinous clot, by adding to it a few drops
of whipped blood, i.e. of blood which has coagulated, or
a little serum of blood. Now if a specimen of pericardial
fluid, which has been thus observed not to clot spon-
taneously, but to clot readily on the addition of blood or
serum, be treated with salt in the same way as described
above for blood plasma, a substance will be thrown down,
which, at firit sight, looks exactly like that thrown down
from blood plasma. But there is a great difference, for
the substance thus obtained from pericardial fluid when
dissolved in water will not clot spontaneously, though its
solutions may be made to clot at any time by the addition
of a little serum, or whipped blood. It too may therefore
be spoken of as an antecedent of fibrin, and indeed it
has received the name oi fibrinogen^ or "fibrin maker."
It is undoubtedly present in the substance thrown down
by salt from blood plasma, but then it is mixed with other
bodies ; and the presence of some or other of these
bodies seems to be the reason why in this case it is con-
verted into fibrin, and so gives a clot. Conversely the
absence of this body or these bodies from pericardial
fluid is the reason why pericardial fluid, or fibrinogen
prepared from pericardial fluid, does not clot spon-
taneously.

Besides fibrinogen there is present in blood plasma,
and thrown down like it by salt, a very similar body
which has been called globulin, or paraglobulin ; and it is
thought by many that fibrinogen is converted into fibrin
by some inter-action between it and paraglobulin. But
serious objections have been urged against this view,
which cannot be regarded as definitely proved. Moreover,
there are reasons for thinking that in the conversion of
fibrinogen into fibrin an important part is played by the
presence in shed blood in very small quantities of a body
belonging to a remarkable class of substances called

72 ELEMENTARY PHYSIOLOGY. [less.

"ferments,"' of which we shall have to speak when we
come to consider digestion. These ferments are charac-
terized by their power, even when present in small quan-
tities, of producing great changes in other bodies without
themselves entering into the changes. Thus the par-
ticular ferment of which we are speaking, and which has
been called "fibrin ferment," produces fibrin, and yet
does not itself become part of the fibrin so produced.

We may say then that fibrin as such does not exist in
the blood at the moment of its being shed, but makes its
appearance afterwards on account of the action of fibrin
ferment on fibrinogen, other bodies as well being possibly
concerned in the matter.

15. The proverb that "blood is thicker than water"' is
literally true, as the blood is not only " thickened " by the
corpuscles, of which it has been'calculated that no fewer
than 70,000,000,000 (eighty times the number of the human
population of the globe) are contained in a cubic inch,
but is rendered slightly viscid by the solid matters dis-
solved in the plasma. The blood is thus rendered heavier
than water, its specific gravity being about i "055. In other
words, twenty cubic inches of blood have about the same
weight as twenty-one cubic inches of water.

The corpuscles are heavier than the plasma, and their
volume is usually somewhat less than that of the plasma.
Of colourless corpuscles there are usually not more than
three or four for every thousand of red corpuscles ; but
the proportion varies very much, increasing shortly after
food is taken, and diminishing in the intervals between
meals.

The blood is hot, its temperature being about 100^
Fahrenheit.

16. Considered chemically, the blood is an alkaline
fluid, consisting of water, of solid and of gaseous matters.

The proportions of these several constituents vary
according to age, sex, and condition, but the following
statement holds good on the average : —

In every 100 parts of the blood there are 79 parts of
water and 21 parts of dry solids ; in other words, the
water and the solids of the blood stand to one another in
about the same proportion as the nitrogen and the oxygen
of the air. Roughly speaking, one quarter of the blood

III.] GASES OF TlIK llLOOD. 73

is dry, solid matter; three quarters water. Of the 21
parts of dry sohds, 12 (= 7-ths) belong to the corpuscles.
The remaining- 9 are about two-thirds (67 parts = fths)
albumin (a substance like white of egg, coagulating by
heat), and one-third (= 7th of the whole solid matter) a
mixture of saline, fatty, and saccharine matters, sundry
products of the waste of the body, and fibrin. The
quantity of the latter constituent is remarkably small in
relation to the conspicuous part it plays in the act of
coagulation. Healthy blood, in fact, yields in coagulating
not more than from two to four parts in a thousand of its
weight of fibrin.

The total quantity of gaseous matter contained in the
blood is equal to rather more than half the volume of the
blood ; that is to say, 100 cubic inches of blood will con-
tain about 60 cubic inches of gases. These gaseous
matters are carbonic acid, oxygen, and nitrogen ; or, in
other words, the same gases as those which exist in the
atmosphere, but in totally different proportions ; for
whereas air contains nearly three-fourths nitrogen, one-
fourth oxygen, and a mere trace of carbonic acid, the
average composition of the blood gases is about two-
thirds or more carbonic acid, and one-third or less oxygen,
the quantity of nitrogen being exceedingly small.

It is important to observe that blood contains much
more oxygen gas than could be held in solution by pure
water at the same temperature and pressure. This
power of holding oxygen appears in some way to depend
upon the corpuscles, firstly, because mere serum has no
greater power of absorbing oxygen than pure water has ;
and secondly, because red corpuscles suspended in water
instead of serum absorb oxygen very readily. The oxygen
thus held by the red corpuscles is readily given up by
them for purposes of oxidation, and indeed can be
removed from them by means of a mercurial gas pump.
It would appear that the connection between the oxygen
and the red corpuscles is of a peculiar nature, being a sort
of loose chemical combination with one of their con-
stituents, that constituent being the haemoglobin ; for
solutions of haemoglobin behave towards oxygen almost
exactly as blood does. Similarly the blood contains more
carbonic acid than could be held in solution by pure

74 ELEMENTARY PIIVSIOLOGV. [less.

water at the same temperature and pressure. But unlike
the oxygen, the carbonic acid thus held by blood is not
peculiarly associated with the haemoglobin of the red
corpuscles ; in fact it seems to be chiefly retained by some
constituents of the serum.

The corpuscles differ chemically from the plasma, in
containing a large proportion of the fats and phosphates,
all the iron, and almost all the potash, of the blood ;
while the plasma, on the other hand, contains by far the
greater part of the chlorine and the soda.

17. The blood of adults contains a larger proportion of
solid constitutents than that of children, and that of men
more than that of women ; but the difference of sex is
hardly at all exhibited by persons of flabby, or what is
called lymphatic, constitution.

Animal diet tends to increase the quantity of the red
corpuscles ; a vegetable diet and abstinence to diminish
them. Bleeding exercises the same influence in a still
more marked degree, the quantity of red corpuscles being
diminished thereby in a much greater proportion than
that of the other solid constituents of the blood.

18. The total quantity of blood contained in the body
varies at different times, and the precise ascertainment of
its amount is very difficult. It may probably be estimated,
on the average, at not less than one-thirteenth of the
weight of the body.

19. The function of the blood is to supply nourishment
to, and take away waste matters from, all parts of the
body. All the various tissues may be said to live on the
blood. From it they obtain all the matters they need,
and to it they return all the waste material for which they
have no longer any use. It is absolutely essential to the
life of every part of the body that it should be in such
relation with a current of blood, that matters can pass
freely from the blood to it, and from it to the blood, by
transudation through the walls of the vessels in which the
blood is contained. And this vivifying influence depends
upon the corpuscles of the blood. The proof of these
statements lies in the following experiments :— If the
vessels of a limb of a living animal be tied in such a
manner as to cut off the supply of blood from the limb,
without affecting it in any other way, all the symptoms of

III.] THE LYMI'II. 75

death will set in. The liinb will grow pale and cold, it
will lose its sensibility, and volition will no longer have
power over it ; it will stiffen, and eventually mortify and
decompose.

But, if the ligatures be removed before the death
stiffening has become thoroughly established and the
blood be allowed to flow into the limb, the stiffening
speedily ceases, the temperature of the part rises, the
sensibility of the skin returns, the will regains power over
the muscles, and, in short, the part returns to its normal
condition.

If, instead of simply allowing the blood of the animal
operated upon to flow again, such blood, deprived of its
fibrin by whipping, but containing its corpuscles, be arti-
ficially passed through the vessels, it will be found nearly
as effectual a restorative as entire blood ; while, on the
other hand, the serum (which is equivalent to whipped
blood without its corpuscles) has no such effect.

It is not necessary that the blood thus artificially in-
jected should be that of the subject of the experiment.
Men, or dogs, bled to apparent death, may be at once and
effectually revived by filling their veins with blood taken
from another man, or dog ; an operation which is known
by the name of transfusion.

Nor is it absolutely necessary for the success of this
operation that the blood used in transfusion should belong
to an animal of the same species. The blood of a horse
will permanently revive an ass, and, speaking generally,
the blood of one animal may be replaced without injurious
effects by that of another closely-allied species ; while
that of a very different animal will be more or less in-
jurious, and may even cause immediate death.

20. The Lymph, which fills the lymphatic vessels, is,
like the blood, an alkaline fluid, consisting of a plasma
and corpuscles, and coagulates by the separation of fibrin
from the plasma. The lymph differs from the blood in
its corpuscles being all of the colourless kind, and in the
very small proportion of its solid constituents, which
amount to only about 5 per cent, of its weight. Lymph
may, in fact, be regarded as blood minus its red cor-
puscles, and diluted with water, so as to be somewhat less

76 ELEMENTARY PHYSIOLOGY. [less.

dense than the serum of blood, which contains about 8
per cent, of sohd matters.

A quantity of fluid equal to that of the blood is pro-
bably poured into the blood, daily, from the lymphatic
system. This fluid is in great measure the mere overflow
of the blood itself — plasma which has exuded from the
capillaries into the tissues, and thus has escaped passing
on into the venous current ; the rest is due to the
absorption of chyle from the alimentary canal.

IV.] RESPIRATION.

LESSON IV.

R ESr/K A r ION,

1, The blood, the general nature and properties of
which liave been described in the preceding Lesson, is
the highly complex product, not of any one organ or con-
stituent of the body, but of all. Many of its features are
doubtless given to it by its intrinsic and proper structural
elements, the corpuscles ; but the general character of
the blood is also profoundly affected by the circumstance
that every other part of the body takes something from
the blood and pours something into it. The blood may
be compared to a river, the nature of the contents of
which is largely determined by that of the head waters,
and by that of the animals which swim in it ; but which
is also very much affected by the soil over which it
flows, by the water-weeds which co^•er its banks, and
by affluents from distant regions ; by irrigation works
which are supplied from it, and by drain-pipes which
flow into it.

2. One of the most remarkable and important of the
changes effected in the blood is that which results, in
most parts of the body, from its simply passing through
capillaries, or, in other words, through vessels the walls
of which are thin enough to permit a free exchange
between the blood and the fluids which permeate the
adjacent tissues (Lesson II. § i).

Thus, if blood be taken from the artery which supplies
a limb, it will be found to have a bright scarlet colour ;
while blood drawn, at the same time, from the vein of the
limb, will be of a purpli^sh hue, so dark that it is com-
monly, called "black blood." And as this contrast is met

78 ELEMENTARY PHYSIOLOGY. [less.

with in the contents of the arteries and veins in general
(except the pulmonary artery and veins), the scarlet blood
is commonly known as arterial and the dark blood as
venous.

This conversion of arterial into venous blood takes
place in most parts of the body, while life persists. Thus,
if a limb be cut off and scarlet blood be forced into its
arteries by a syringe, it will issue from the veins as dark
blood.

3. When specimens of venous and of arterial blood are
subjected to chemical examination, the differences pre-
sented by their solid and fluid constituents are found to be
very small and inconstant. But the gaseous contents of
the two kinds of blood differ widely in the proportion
which the carbonic acid gas bears to the oxygen ; there
being a smaller quantity of oxygen and a greater quantity
of carbonic acid, in venous than in arterial blood.

And it may be experimentally demonstrated that this
difference in their gaseous contents is the only essential
difterence between venous and arterial blood. For if
venous blood be shaken up with oxygen, or even with air,
it gains ox}gen, loses carbonic acid, and takes on the
colour and properties of arterial blood. Similarly, if
arterial blood be treated with carbonic acid so as to be
thoroughly saturated with that gas, it gains carbonic acid,
loses oxygen, and acquires the true properties of venous
blood ; though, for a reason to be mentioned below, the
change is not so complete in this case as in the fomier.
The same result is attained, though more slowly, if the
blood, in either case, be received into a bladder, and then
placed in the carbonic acid, or oxygen gas ; the thin
moist animal membrane allowing the change to be effected
with perfect ease, and offering no serious impediment to
the passage of either gas.

4. The physico-chemical processes involved in the
exchange of carbonic acid for oxygen, when venous is
converted into arterial blood, or the reverse, in the cases
mentioned above, are of a somewhat complex nature.

It is known ia) that gases, mechanically held by a fluid
in a given proportion, tend to diffuse into any atmosphere
to which they are exposed until they occupy that atmo-
sphere in corresponding proportions ; and ip) that gases

IV.] ARTERIAL AND VENOUS BLOOD. 79

separated by a dry porous partition, or simply in contact,
diffuse into one another with a rapidity which is inversely
proportioned to the square roots of their densities. A
knowledge of these physical principles does, in a rough
way, lead us to see how the gases contained in the blood
may effect an exchange with those in the air, whether the
blood be freely exposed, or inclosed in a membrane.

But the application of these principles gives no more
than this sort of general insight For, in the first place,
when arterialization takes place through the walls of a
bladder, or any other thin animal membrane, the matter
is complicated by the circumstance that moisture dissolves
carbonic acid far more freely than it will oxygen ; hence
a wet bladder has a very different action upon carbonic
acid from that which it has upon oxygen. A moist
bladder, partially filled with oxygen, and suspended in
carbonic acid gas, becomes rapidly distended, in con-
sequence of the carbonic acid gas passing into it with
much greater rapidity than the oxygen passes out.
Secondly, the gases of the blood are not held in a
merely mechanical way in it ; the oxygen seems to be
loosely combined with the red corpuscles (Lesson IIL
§ 16}, and there is reason to think that a great part, at
least, of the carbonic acid, is chemically connected, in a
similarly loose way. with certain saline constituents of
the serum. Hence the arterialization of blood in the
lungs seems to be a ver}- mixed process, partly physical,
and yet, to a certain extent chemical, and consequently
very difficult to analyse.

The same may also be said of the change from arterial
to venous blood in the tissues. Owing to the peculiar
relation of oxygen to the red blood-corpuscles, the process
which takes place in the tissues is not a simple inter-
change by diffusion of the oxygen of the blood for the
carbonic acid of the tissues ; on the contrar}-, the oxygen
is given up for purposes of oxidation, the demand being
detemiined by the activity of the tissue, while the blood,
poor in carbonic acid, takes up, apparently by an in-
dependent action, a quantity of that gas from the tissues
rich in it.

Hence venous blood is characterized not only by the
large amount of carbonic acid present, but also by the

8o ELEMENTARY PHYSIOLOGY. [less.

fact that the red corpuscles have given up a good deal of
their oxygen for the purposes of oxidation, or, as the
chemists would say, have become reduced. This is the
reason why arterial blood is not so easily converted into
venous blood by exposure to carbonic acid as venous
blood into arterial by exposure to oxygen. There is, in
the former case, a want of some oxidizable substance to
carry off the oxygen from and so to reduce the red cor-
puscles. When such an oxidizable substance is added
(as, for instance, a salt of iron) the blood at once and
immediately becomes completely venous.

Practically we may say that the most important differ-
ence between venous and arterial blood is not so much
the relative quantities of carbonic acid as that the red cor-
puscles of venous blood have lost a good deal of oxygen,
are reduced, and ready at once to take up any oxygen
offered to them.

5. Similarly the loss of oxygen by the red corpuscles is
the chief reason why the scarlet arterial blood turns
of a more purple or claret colour in becoming venous.
It has indeed l3een urged that the red corpuscles are
rendered somewhat flatter by oxygen gas, while they are
distended by the action of carbonic acid (Lesson IIL § 4).
Under the former circumstances they may, not improbably,
reflect the light more strongly, so as to give a more
distinct coloration to the blood ; while, under the latter,
they may reflect less light, and, in that way, allow the
blood to appear darker and duller.

This, however, can only be a small part of the whole
matter ; for solutions of haemoglobin or of blood-crystals
(Lesson IIL § 9), even when perfectly free from actual
blood-corpuscles, change in colour from scarlet to purple,
according as they gain or lose oxygen. It has already
been stated (Lesson III. § 16), that oxygen most probably
exists in the blood in loose combination with haemo-
globin. And further, there is evidence to show that a
solution of haemoglobin, when thus loosely combined with
oxygen, has a scarlet colour, while a solution of haemo-
globin deprived of oxygen has a purplish hue. Hence
arterial blood, in which the haemoglobin is richly pro-
vided with oxygen, would naturally be scarlet, while
venous blood, which not only contains an excess of

IV.] ARTERIAL AND VENOUS BLOOD. ' Si

carbonic acid, but whose haemoglobin also has lost a
great deal of its oxygen, would be purple.

6, Now all the tissues, as we have seen, are continually
using up oxygen. Their life in fact is dependent on a
continual succession of oxidations. Hence they are greedy
of oxygen, while at the same time they are continually
producing carbonic acid (and other waste products). Thus,
as the blood is flowing through the capillaries of a tissue
we have on the one side of the permeable capillary wall
the blood with its corpuscles rich in oxygen, and on the
other side the tissue in constant want of oxygen, and
constantly producing a large quantity of carbonic acid.
The result is that the oxygen flies from the red corpuscles
through the capillary wall to the tissue, which at once
takes it up, while the carbonic acid passes from the tissue
where it is in excess through the capillary wall to the
blood which, though containing carbonic acid, does not
hold so much as the tissue. The blood therefore leaves
the tissue poorer in oxygen and richer in carbonic acid
than when it came to it ; and this change is the change
from the arterial to the venous condition.

On the other hand, if we seek for the explanation of
the conversion of the dark blood in the veins into the
scarlet blood of the arteries, we find, ist, that the blood
remains dark in 'the right auricle, the right ventricle, and
the pulmonary artery ; 2nd, that it is scarlet not only in
the aorta, but in the left ventricle, the left auricle, and
the pulmonary veins.

Obviously, then, the change from venous to arterial
takes place in the pulmonan* capillaries, for these are the
sole channels of communication between the pulmonar}-
arteries and the pulmonan,- veins.

7. But what are the physical conditions to which the
blood is exposed in the pulmonar\- capillaries ?

These vessels are very wide, thin walled, and closely set,
so as to form a network with ver)' small meshes, which is
contained in the substance of an extremely thin mem-
brane. This membrane is in contact with the air, so that
the blood in each capillar}' of the lung is separated from
the air by only a delicate pellicle formed by its own wall
and the lung membrane. Hence an exchange very readily
takes place between the blood and the air : the latter

82 ELEMENTARY PHYSIOLOGY. [less.

gaining moisture and carbonic acid, and losing oxygen
(Lesson L §§ 23, 24).!

This is the essential step ni respiration. That it really
takes place may be demonstrated very readily, by the
experiment described in the first Lesson (§ 3), in which
air expired was proved to ditfer from air inspired, by con-
taining more heat, more water, more carbonic acid, and
less oxygen ; or, on the other hand, by putting a ligature
on the windpipe of a living animal so as to prevent air
from passing into, or out of, the lungs, and then examin-
ing the contents of the heart and great vessels. The
blood on both sides of the heart, and in the pulmonary
veins and aorta, will then be found to be as completely
venous as in the venas cavje and pulmonary artery.

But though the passage of carbonic acid gas and hot
water}' vapour out of the blood and of oxygen into it
is the essence of the respiratory process — and thus a
membrane with blood on one side, and air on the other,
is all that is absolutely necessary to effect the purification
of the blood — yet the accumulation of carbonic acid is so
rapid, and the need for oxygen so incessant, in all parts
of the human body, that the former could not be cleared
away, nor the latter supplied, with adequate rapidity,
without the aid of extensive and complicated accessory
machinery — the arrangement and working of which must
next be carefully studied.

8. The back of the mouth or phary?ix communicates
by two channels with the external air (see Fig. 40). One
of these is formed by the nasal passages, which cannot
be closed by any muscular apparatus of their own ; the
other is presented by the mouth, which can be shut or
opened at will.

Immediately behind the tongue, at the lower and front
part of the phar>'nx, is an aperture — the glottis (Fig.
19 Gl) — capable of being closed by a sort of lid — the
epiglottis — or by the shutting together of its side bound-
aries, formed by the so-called vocal chords. The glottis

' The student must guard himself against the idea that arterial blood con-
tains no carbonic acid, and venous blood no oxygen. In passing through the
lungs venous bbod loses only apart of its carbonic acid ; and arterial blood,
in passing through the tissues, loses only a part of its o.xj'gen. In blood,
however venous, there is in health always sume oxygen; and in even the
brightest arterial bbod there is actually more carbonic acid than oxygen.

IV.]

THE AIR PASSAGES.

opens into a chamber with cartilaginous walls — the
larynx ; and leading from the larynx downwards along
the front part of the throat, where it may be very readily
felt, is the trachea^ or windpipe (Fig. 19, />}.

V.C.L

Fig. 19. — Back View of the Xeck and Thor.a.xof a Human Slbject

FROM WHICH THE VERTEBRAL COLCMN AND WHOLE POSTERIOR WaLL

OF THE Chest are supposed to be removed.

J/, mouth ; G/, glottis ; Tr, trachea ; L.L, left lung ; R.L, right lung ; Br,
bronchus; P. A, pulmonary artery*; P.V, pulmonary veins; Ao, aorta;
D, diaphragm ; H, heart ; V.C.I, vena cava inferior.

If the trachea be handled through the skin, it will be
found to be firm and resisting. Its walls are, in fact,
strengthened by a series of cartilaginous hoops, which
hoops are incomplete behind, their ends being united
only by muscle and membrane, where the trachea comes
into contact with the gullet, or a'sophagus. The trachea
passes into the thorax, and there divides into two branches,
a right and a left, which are termed the brofichi (Fig.
19, Br). Each bronchus enters the lung of its own side,

G 2

84 ELEMLNTARV PHYSIOLOGY. [less.

and then breaks up into a great number of smaller
branches, which are called the b}'07ichial tubes. \s these
diminish in size, the cartilages, which are continued all
through the bronchi and their large ramirications, become
smaller and eventually disappear, so that the walls of the
smallest bronchial tubes are entirely muscular or mem-
branous. Thus while the trachea and bronchi are kept
permanently open and pervious to air by their cartilages,
the smaller bronchial tubes may be almost closed by the
contraction of their muscular walls.

The finer bronchial tubes end at length in elongated
dilatations, about ^V^h of an inch in diameter on the average
(Fig. 20, A). Each of these dilatations is beset with,
or perhaps rather is made up of, little sacs, which open
irregularly into the cavity of the dilatation. These sacs
are the air-cells. The very thin walls (Fig. 20, B) which
separate these air-cells are supported by much delicate
and highly elastic tissue, and carry the wide and close-set
capillaries into which the ultimate ramifications of the
pulmonary artery pour its blood (Tig. 20, D). Thus, the
blood contained in these capillaries is exposed on both
sides to the air — being separated from the air-cell on
either hand only by the very delicate pellicle which forms
the wall of the capillary, and the lining of the air-sac.

9. Hence no conditions can be more favourable to a
ready exchange between the gaseous contents of the blood
and thoEe of the air in the air-cells, than the arrangements
which obtain in the pulmonary capillaries ; and, thus far,
the structure of the lung fully enables us to understand
how it is that the large quantity of blood poured through
the pulmonary circulation becomes exposed in very thin
streams, over a large surface, to the air. But the only
result of this arrangement would be, that the pulmonary
air would very speedily lose all its oxygen, and become
completely saturated with carbonic acid, if special pro-
vision were not made for its being incessantly renewed.

10. If an adult man, breathing calmly in the sitting
position, be watched, the respiratory act will be observed
to be repeated thirteen to fifteen times every minute.
Each act consists of certain components which succeed
one another in a regular rhythmical order. First, the
breath is drawn in, or inspired ; immediately afterwards

IV.]

INSPIRATION AND EXPIRATION.

it is driven out, or expired ; and these successive acts of
ifispiraiion and expiration are followed by a brief pause.
Thus, just as in the rhythm of the heart the auricular
systole, the ventricular systole, and then a pause, follow in

Fig. 2o.

A. Two air-cells (i'') with the ultimate bronchial tube («) which opens into

them. (Magnified 20 diameters.)

B. Diagrammatic view of an air-cell of A seen in section : a, epithelium ;

b, partition between two adjacent cells, in the thickness of which the

capillaries run ; c, fibres of elastic tissue.
C Portion of injected lung magnified : a, the capillaries spread over the walls

of two adjacent air-cells ; b, small branches of arteries and veins.
D. Portion still more highly magnified.

86 ELEMENTARY PHYSIOLOGY. [less.

regular order ; so in the chest, the inspiration, the expi-
ration, and then a pause succeed one another. But in
the chest, unhke the case of the heart, the pause is
generally very short compared with the active movement ;
indeed, sometimes it hardly exists at all, a new inspiration
following immediately upon the close of expiration. At
each inspiration of an adult well-grown man about thirty
cubic inches of air are inspired ; and at each expiration
the same, or a slightly smaller, volume (allowing for the
increase of temperature of the air so expired) is given out
of the body.

11. The expired air differs from the air inspired in the
following particulars : —

(a) Whatever the temperature of the external air is, that
expired is nearly as hot as the blood, or has a temperature
between 98'' and 100°.

(d) However dry the external air maybe, that expired is
quite, or nearly, saturated with watery vapour.

(c) Though ordinary air contains nearly 2,100 parts of
oxygen, and 7,900 of nitrogen, with not more than 3 parts
of carbonic acid, in 10,000 parts, expired air contains
about 470 parts of carbonic acid, and only between 1,500
and 1,600 parts of ox)gen ; while the quantity of nitrogen
suffers little or no change. Speaking roughly, air which
has been breathed once has gained five per cent, of
carbonic acid, and lost five per cent, of oxygen.

The expired air contains, in addition, a greater or less
quantity of animal matter of a highly decomposable
character.

(d) Yer}' close analysis of the expired air shows, firstly,
that the quantity of oxygen which disappears is always
slightly in excess of the quantity of carbonic acid sup-
plied ; for all the oxygen taken in does not go to form
carbonic acid, some of it is employed to unite with
hydrogen (forming water), and indeed with other ele-
ments ; and secondly, that the nitrogen is variable — the
expired nitrogen being sometimes slightly in excess of,
sometimes slightly less than, that inspired, and sometimes
remaining stationary.

12. From three hundred and fifty to four hundred cubic
feet of air are thus passed through the lungs of an adult
man taking little or no exercise, in the course of twenty-

IV.] THE THORAX. 87

four hours ; and arc charged with carbonic acid, and
deprived of oxygen, to the extent of nearly five per cent
This amounts to about eighteen cubic feet of the one gas
taken in, and of the other given out. Thus, if a man be
shut up in a close room, having the form of a cube seven
feet in the side, every particle of air in that room will
have passed through his lungs in twenty-four hours, and
a fourth of the oxygen it contained will be replaced by
carbonic acid.

The quantity of carbon eliminated in the twenty-four
hours is pretty nearly represented by a piece of pure char-
coal weighing eight ounces.

The quantity of water given off from the lungs in the
twenty-four hours varies very much, but may be taken on
the average as rather less than half a pint, or about nine
ounces. It may fall below this amount, or increase to
double or treble the quantity.

13. The mechanical arrangements by which the respi-
ratory movements, essential to the removal of the great
mass of effete matters, and the importation of the large
quantity of oxygen indicated, are effected, may be found
in — (a) the elasticity of the lungs ; {l>) the mobility of the
sides and bottom of the thoracic cavity in which the lungs
are contained.

The thorax may be regarded as a completely shut coni-
cal box, with the small end turned upwards, the back of
the box being formed by the spinal column, the sides by
the ribs, the front by the breast-bone, the bottom by the
diaphragm, and the top by the root of the neck (Fig. 19).

The two lungs occupy almost all the cavity of this box
which is not taken up by the heart. Each is enclosed in
its serous membrane, the _p/t-ura, a double bag (very simi-
lar to the pericardium, the chief difference being that the
outer bag of each pleura is, over the greater part of its ex-
tent, quite tirmly adherent to the walls of the chest and
the diaphragm (see Fig. 9), while the outer bag of the peri-
cardium is for the most part loosed the inner bag closely
covering the lung and the outer forming a lining to the
cavity of the chest. So long as the walls of the thorax
are entire, the cavity of each pleura is practically oblite-
rated, that layer of the pleura which covers the lung being
in close contact with that which lines the wall of the

ELEMENTARY PHYSIOLOGY.

[less.

chest ; but if a small opening be made into the pleura,
the lung at once shrinks to a comparatively small size,
and thus develops a great cavity between the two layers
of the pleura. If a pipe be now fitted into the bronchus,

Fig. 21.— Vie\v of Four Ribs of the Dog with the Ln'tercostal

Muscles.

a, the bony rib; h, the cartilage; r, the junction of bone and cartilage;
d, unossified, e, ossified, portions of the sternum. A, external intercostal
muscle. B, internal intercostal muscle. In the middle interspace, the
external intercostal has been removed to show the internal intercostal
beneath it.

and air blown through it, the lung is very readily dis-
tended to its full size ; but, on being left to itself, it col-
lapses, the air being driven out again with some force.
The abundant elastic tissues of the walls of the air-cells
are, in fact, so disposed as to be greatly stretched when
the lungs are full ; and, when the cause of the distension

iv.] THE ELASTICITY OF THE LUNG'S. C9

is removed, tjiis elasticity comes into play and drives the
greater part of the air out again.

The lungs arc kept distended in the dead subject, so
long as the walls of the chest are entire, by the pressure
of the atmosphere. For though the elastic tissue is all
the while pulling, as it were, at the layer of pleura which
covers the lung, and attempting to separate it from that
which lines the chest, it cannot produce such a separation
without developing a vacuum between these two layers.
To effect this, the elastic tissue must pull with a force
greater than that of the external air (or fifteen pounds to
the square inch), an effort far beyond its powers, which
do not equal more than one-fourth of a pound on the
square inch. But the moment a hole is made in the
pleura, the air enters into its cavity, the atmospheric pres-
sure inside the lung is equalized by that outside it, and
the elastic tissue, freed from its opponent, exerts its full
power on the lung.

14. The lungs are elastic, whether alive or dead. During
life the air which they contain may be further affected by
the contractility of the muscular walls of the bronchial
tubes. If water is poured into the lungs of a recently-
killed animal, and a series of electric shocks is then sent
through the bronchial tubes, the latter contract, and the
water is forced out. Lastly, during life a further source
of motion in the bronchial tubes is provided by the cilia
— minute filaments attached to the epithelium of the tubes,
which incessantly vibrate backwards and forwards, and
work in such a manner as to sweep liquid and solid matters
outwards, or towards the trachea. But these cilia have
practically no effect on the movement of the air in the
lungs, and the contractions of the muscular walls of
the bronchi are probably made use for special purposes
only,

15. The ribs are attached to the spine, so as to be freely
moveable upon it ; but when left to themselves they take
a position which is inclined obliquely downwards and
forwards.^ Tvro sets of muscles, called intercostah^ pass

^ I purp:sely neglect the consideration of the cartilages of the ribs, and
some other points, in order not to complicate the question unnecessarily. It
may, however, be stated that those fibres of the internal intercostals which
are situated between the cartilages act probably like the external, and rais?
the ribs.

ELEMENTARY PHYSIOLOGY.

[less.

between the successive pairs of ribs on eaph side. The
outer set, c2\\Q.(lcx/crfial intercostals {¥\^. 21,^), run from
the rib above, obHquely downwards and forwards, to the
rib below. The other set, internal intcrcostals (Fig. 21, B)y
cross these in direction, passing from the rib above, down-
wards and backwards, to the rib below.

The action of these muscles is somewhat puzzling at
first, but is readily understood if the fact that ivhen a
muscle contracts^ it tends to sJiorteii the distance between its
two ends be borne in mind. Let a and b in Fig. 22, A,
be two parallel bars, moveable by their ends upon the

Fig. 22. — Diagram of Models illustrating the Action of the
External and Internal Intercostal Muscles.

B, inspiratory elevation ; C, expiratory depression.

upright c, which may be regarded as at the back of the
apparatus, then a line directed from x to _y will be inclined
downwards and forwards, and one from w to x- will be
directed downwards and backwards. Now it is obvious
from the figure that the distance between x and y is
shorter in B than in A and much shorter than in C ;
hence when x y is shortened the bars will be pulled up
from the position C or A to or towards the position B.
Conversely the shortening oi iv z will tend to pull the
bars down from the position B or the position A to or
towards the position C.

IV.] THE DIAPHRAGM. gr

If the simple apparatus just described be made ot wood,
hooks being placed at the points xy, and w .^ ; and an
elastic band be provided with eyes which can ba readily
put on to or taken off these hooks ; it will be found that
the band being so short as to be put on the stretch when
hooked on to either x y, or w z, with the bars in the
horizontal position, A, the elasticity of the band, when
hooked on to x and_>', will bring them up as shown in B ;
while, if hooked on to tv and s, it will bring them down
as shown in C.

Substitute the contractility of the external and internal
intercostal muscles for the shortening of the band, in
virtue of its elasticity, and the model will exemplify the
action of these muscles ; the external intercostals in
shortening will tend to raise, and the internal intercostals
to depress, the bony ribs.

Such a model, however, does not accurately represent
the ribs, with their numerous and peculiar cur^•es, and
hence, while all are agreed that the external intercostals
raise the ribs, the action of the internal intercostals is
not quite so certain.

i6. The diaphragm is a great partition situated between
the thorax and the abdomen, and always concave to the
latter and convex to the former (Fig. i, D). From its
middle, which is tendinous, muscular fibres extend down-
wards and outwards to the ribs, and two, especially
strong masses, which are called the pillars of the dia-
phragm, to the spinal column (Fig. 23). When these
muscular fibres contract, therefore, they tend to make the
diaphragm flatter, and to increase the capacity of the
thorax at the expense of that of the abdomen, by pulling
down the bottom of the thoracic box (Fig. 24, A).

17. Let us now consider what would be the result of
the action of the parts of the respirator)- apparatus which
have been described, if the diaphragm alone should begin
to contract at regular intervals.

When it contracts it increases the vertical dimensions
of the thoracic cavit}-, and tends to pull away the lining of
the bottom of the thoracic box from that which covers the
bases of the lungs ; but the air immediately rushing in at
the trachea, proportionately increases the distension of
the lungs, and prevents the formation of any vacuum

ELEMENTARY rHVSIOLOGV

[less.

between the two pleurae of either lung in this region. When
the diaphragm ceases to contract, so much of the elasticity
of the lungs as was neutralized by the contraction of the
diaphragm, comes into play, and the extra air taken in is
driven out again. Wc have, in short, an Inspiration and
an Expiratio7i.

Fig. 23. — The Diaphragm of a Dog viewed fro.m the Lower cr
Abdominal Side.

V.C.I, the vena cava inferior ; O, the oesophagus ; Ao, the aorta ; the broad
white tendinous middle (5) is eas.ly distinguished from the radiating
muscular fibres {A) which pass dawn to the ribs and into the pillars (C D)
in front of the vertebrae.

Suppose on the other hand, that, the diaphragm being
quiescent, the external intercostal muscles contract. The
ribs will be raised from their oblique position, the an-
tero-posterior dimensions of the thoracic cavity will be

iv.] ACCESSORY MUSCLES OF RESPIkATION. 93

increased (for each rib, as it moves from the slanting to the
horizontal position, must thrust the sternum outwards),
and the lungs will be distended as before to balance
the enlargement. If now the external intercostals relax,
the action of gravity upon the ribs, the elasticity of the
cartilages, and more especially that of the lungs, will alone
suffice to bring back the ribs to their previous positions
and to drive out the extra air ; and this expiratory
action may be aided by the contraction of the internal
intercostals.

18. Thus it appears that we may have either diaphrag-
matic respiration^ or costal rcspiratioji. As a general rule,
however, not only do the two forms of respiration coincide
and aid one another — the contraction of the diaphragm
taking place at the same time with that of the external
intercostals, and its relaxation with their relaxation —
but sundry other accessory agencies come into play.
Thus, the muscles which connect the ribs with parts of
the spine above them, and with the shoulder, may, more
or less extensively, assist inspiration, especially certain
muscles which pull up and fix the first two ribs and so
allow the whole force of each external intercostal muscle to
be spent in raising the rib below it ; while those which
connect the ribs and breast-bone with the pelvis, and form
the front and side walls of the abdomen, are powerful
aids to expiration. In fact they assist expiration in two
ways : first, directly, by pulling down the ribs ; and next,
indirectly, by pressing the viscera of the abdomen
upwards against the under surface of the diaphragm,
and so driving the lloor of the thorax upwards.

It is for this reason that, whenever a violent expirator}*
effort is made, the walls of the abdomen are obviously
flattened and driven towards the spine, the body being at
the same time bent forwards.

In taking a deep inspiration, on the other hand, the
walls of the abdomen are relaxed and become convex, the
viscera being driven against them by the descent of the
diaphragm— the spine is straightened, the head thrown
back, and the shoulders outwards, so as to attbrd the
greatest mechanical advantage to all the muscles which
can elevate the ribs.

19. It is a remarkable circumstance that the mechanism

94 ELEMENTARY PHYSIOLOGY. [less.

of respiration is somewhat different in the two sexes. In
men, the diaphragm takes the larger share in the process,
the upper ribs moving comparatively Httle ; in women, the
reverse is the case, the respiratory act being more largely
the result of the movement of the ribs.

Sighifig is a deep and prolonged inspiration. " SmJJifig"
is a more rapid inspiratory act, in which the mouth is kept
shut, and the air made to pass through the nose.

Coughhig is a violent expiratory act. A deep inspira-
tion being tirst taken, the glottis is closed and then burst
open by the violent compression of the air contained in
the lungs by the contraction of the expiratory muscles,
the diaphragm being relaxed and the air driven through
the mouth. In s?ieezing^ on the contrary, the cavity of the
mouth being shut off from the pharynx by the approxima-
tion of the soft palate and the base of the tongue, the air
is forced through the nasal passages.

20. It thus appears that the thorax, the lungs, and the
trachea constitute a sort of bellows without a valve, in
which the thorax and the lungs represent the body of the
bellows, while the trachea is the pipe ; and the effect of
the respiratory movements is just the same as that of
the approximation and separation of the handles of the
bellows, which drive out and draw in the air through
the pipe. There is, however, one difference between the
bellows and the respiratory apparatus, of great im-
portance in the theory of respiration, though frequently
overlooked ; and that is, that the sides of the bellows can
be brought close together so as to force out all, or nearly
all, the air which they contain ; while the walls of the
chest, when approximated as much as possible, still inclose
a very considerable cavity (Fig. 24, B) ; so that, even
after the most violent expiratory effort, a very large
quantity of air is left in the lungs.

The amount of this air which cannot be got rid of, and
is called Residual air, is, on the average, from 75 to 100
cubic inches.

About as much more in addition to this remains in
the chest after an ordinary expiration, and is called
Supple))iental air.

In ordinary breathing, 20 to 30 cubic inches of what is
conveniently called lidal air ^a.ss in and out. It follows

IV.]

TIDAL AIR.

that, after an ordinary inspiration, loo + loo + 30 = 230
cubic inches, may be contained in the lungs. By taking
the deepest possible inspiration, another 100 cubic inches,
called Complcnicntal ah', may be added.

21. It results from these data that the lungs, after an
ordinary inspiration, contain about 230 cubic inches of

Fig. 24.— Diagrammatic Sections cf the Body in

A inspiration ; B, expiration. Tr, trachea ; St, sternum ; D, diaphragm ;

'Ab, abdominal woXls,. The shading roughly indicates the stationary air.

air, and that only about one-seventh to one-eighth of this
amount is breathed out and taken in again at the next
inspiration. Apart from the circumstance, then, that the
fresh air inspired has to fill the cavities of the hinder part
of the mouth, and the trachea, and the bronchi, if the
lungs were mere bags fixed to the ends of the bronchi, the

96 ELEMEXTARV PHYSIOLOGY. [less.

inspired air would descend so far only as to occupy that
one-fourteenth to one-sixteenth part of each bag which
was nearest to the bronchi, whence it would be driven out
again at the next expiration. But as the bronchi branch
out into a prodigious number of bronchial tubes, the
inspired air can only penetrate for a certain distance
along these, and can never reach the air-cells at all.

Thus the residual and supplemental air taken together
arc, under ordinary circumstances, sfatiotiary — that is to
say, the air comprehended under these names merely
shifts its outer limit in the bronchial tubes, as the chest
dilates and contracts, without leaving the lungs ; the tzdal
air, alone, being that which leaves the lungs and is re-
newed in ordinary respiration.

It is obvious, therefore, that the business of respiration
is essentially transacted by the stationary air, which plays
the part of a middleman between the two parties — the
blood and the fresh tidal air — who desire to exchange
their commodities, carbonic acid for oxygen, and oxygen
for carbonic acid.

Now there is nothing interposed between the fresh tidal
air and the stationary air ; they are aeriform fluids, in
complete contact and continuity, and hence the exchange
between them must take place according to the ordinary
laws of gaseous diffusion.

2 2. Thus, the stationary air in the air-cells gives up
oxygen to the blood, and takes carbonic acid from it,
though the exact mode in which the change is effected is
not thoroughly understood. By this process it becomes
loaded with carbonic acid, and deficient in oxygen,
though to what precise extent is not known. But there
must be a very much greater excess of the one, and
deficiency of the other, than is exhibited by inspired air,
seeing that the latter acquires its composition by diffusion
in the short space of time (four or five seconds) during
which it is in contact with the stationary air.

In accordance with these facts, it is found that the air
expired during the first half of an expiration contains
less carbonic acid than that expired during the second
half. Further, when the frequency of respiration is in-
creased without altering the volume of each inspiration,
though the percentage of carbonic acid in each inspiration

iv.] NERVOUS APPARATUS OF RESPIRATKjX. 97

is diminished, it is not diminished in the same ratio as
that in which the number of inspirations increases ; and
hence more carbonic acid is got rid of in a given time.

Thus, if the number of inspirations per minute is in-
creased from fifteen to thirty, the percentage of carbonic
acid evolved in the second case remains more than half
of what it was in the first case, and hence the total
evolution is greater.

23. Of the various mechanical aids to the respiratory
process, the nature and workings of which have now been
described, one, the elasticity of the lungs, is of the nature
of a dead, constant force. The action of the rest of the
apparatus is under the control of the ner\-ous system, and
varies from time to time.

As the nasal passages cannot be closed by their own
action, air has always free access to the phar}-nx ; but
the glottis, or entrance to the windpipe, is completely
under the control of the nervous system — the smallest
irritation about the mucous membrane in its neighbour-
hood being conveyed, by its nerves, to that part of the
cerebro-spinal axis which is called the medulla oblongata
(see Lesson XI. § 16). The medulla oblongata, thus
stimulated, gives rise, by a process which will be explained
hereafter, termed reflex action^ to the contraction of the
muscles which close the glottis, and commonly, at the
same time, to a violent contraction of the expirator}'
muscles, producing a cough (see § 19). The muscular
fibres of the smaller bronchial tubes are similarly under
the control of the medulla oblongata, sometimes contract-
ing so as to narrow and sometimes relaxing so as to
permit the widening of the bronchial passages.

24. These, however, are mere incidental actions. The
whole respirator}^ machiner)^ is worked by a nervous
apparatus. From what has been said, it is obvious that
there are many analogies between the circulator)- and the
respirator}- apparatus. Each consists, essentially, of a
kind of pump which distributes a fluid (aeriform in the
one case, liquid in the other) through a series of ramified
distributing tubes to a system of cavities (capillaries or
air-cells), the volume of the contents of which is greater
than that of the tubes. While the heart however is a force-
pump, the respirators-machinery represents a suction-pump.

98 ELEMENTARY PHYSIOLOGY. [less.

In each, the pump is the cause of the motion of the
fluid, though that motion may be regulated, locally, by
the contraction or relaxation, of the muscular fibres
contained in the walls of the distributing tubes. But,
while the rhythmic movement of the heart chiefly depends
upon a nervous apparatus placed within itself, that of the
respiratory apparatus results mainly from the operation of
a nervous centre lodged in the medulla oblongata, which
has been called the respiratory centre.

The intercostal muscles are supplied by i?7tercostal nerxes
coming from the spinal cord in the region of the back,
and the muscular fibres of the diaphragm are supplied
by two nerves, one on each side, called the phre?iic nerves,
which starting from certain of the spinal nerves in the
neck, dip into the thorax at the root of the neck, and find
their way through the thorax by the side of the lungs to
the diaphragm, over which they are distributed. Now
from the nervous respiratory centre in the medulla
oblongata, impulses at repeated intervals descend along
the upper part of the spinal cord and, passing out by the
phrenic and intercostal nerves respectively, reach the
diaphragm and the intercostal muscles. These im-
mediately contract, and thus an inspiration takes place.
Thereupon the impulses cease, and are replaced by other
impulses, which though starting from the same centre pass,
not to the diaphragm and external intercostal muscles,
but to other, expiratory, muscles, which they throw into
contraction, and thus expiration is brought out. As a
general rule the inspiratory impulses are much stronger
than the expiratory ; indeed, in ordinary quiet breathing
expiration is chiefly brought about, as we have seen, by
the elasticity of the lungs and chest walls ; these need no
nervous impulses to set them at work, as soon as the
inspiratory impulses cease and the diaphragm and other
inspiratory muscles leave off contracting, they come of
themselves into action. But, in laboured breathing, very
powerful expiratory impulses may leave the medulla and
pass to the various muscles whose contractions help to
drive the air out of the chest.

The implilses, both inspiratory and expiratory, which are
thus started in the medulla, seem to be generated there
in a particular way their rapidity and force appearing to

IV.] RESPIRATORY SOUNDS. 99

be dependent on the condition of the blood which is cir-
culating in the capillaries of the medulla. When the
blood flowing through the medulla becomes more venous,
i.e. contains less oxygen, the impulses are increased, when
it becomes less venous they are diminished. But the
character of these respiratory impulses is also determined,
in a reflex manner, by impulses passing up to the medulla
from the lungs by the pneumogastric nerves, and also by
impulses reaching the medulla from other parts of the
body along other nerves. Thus, when both pneumo-
gastrics are divided, so that no impulses reach the medulla
from the lungs, respiration becomes much slower. And,
as is well known, a dash of cold water on the skin makes
one draw a deep breath or gasp, owing to the impulses
which pass up to the medulla from the part of the skin
affected by the cold water.

25. As there are certain secondary phenomena which
accompany, and are explained by, the action of the heart,
so there are secondary phenomena which are similarly re-
lated to the working of the respiratory apparatus. These
are — {a) the respiratory sounds, and ip) the effect of the in-
spiratory and expiratory movements upon the circulation.

26. The respiratory sotaids or iniirmiirs are audible
when the ear is applied to any part of the chest which
covers one or other of the lungs. They accompany
inspiration and expiration, and very much resemble the
sounds produced by breathing through the mouth, when
the lips are so applied together as to leave a small
interval. Over the bronchi the sounds are louder than
over the general surface. It would appear that these
sounds are produced by the motion of the air along the
air-passages.

27. In consequence of the elasticity of the lungs, a
certain force must be expended in distending them, and
this force is found experimentally to become greater and
greater the more the lung is distended ; just as, in stretch-
ing a piece of india-rubber, more force is required to
stretch it a good deal than is needed to stretch it only a
little. Hence, when inspiration takes place, and the lungs
are distended with air, the heart and the great vessels in
the chest are subjected to a less pressure than are the
blood-vessels of the rest of the body.

H 2

loo ELEMENTARY PHYSIOLOGY. [less.

For the pressure of the air contained in the lungs is
exactly the same as that exerted by the atmosphere upon
the surface of the body ; that is to say, fifteen pounds on
the square inch. But a certain amount of this pressure
exerted by the air in the lungs is counterbalanced by the
elasticity of the distended lungs. Say that in a given
condition of inspiration a pound ^ pressure on the square
inch is needed to overcome this elasticity, then there will
be only fourteen pounds pressure on every square inch
of the heart and great vessels. And hence the pressure
on the blood in these vessels will be one pound per square
inch less than that on the veins and arteries of the rest of
the body. If there were no aortic, or pulmonary, valves,
and if the composition of the vessels, and the pressure
upon the blood in them, were everywhere the same, the
result of this excess of pressure on the surface would be
to drive all the blood from the arteries and veins of the
rest of the body into the heart and great vessels contained
in the thorax. And thus the diminution of the pressure
upon the thoracic blood cavities produced by inspiration,
would, practically, suck the blood from all parts of the
body towards the thorax. But the suction thus exerted,
while it hastened the flow of blood to the heart in the
veins, would equally oppose the flow from the heart to the
arteries, and the two eftects would balance one another.

As a matter of fact, however, we know —

(i.) That the blood in the great arteries is constantly
under a very considerable pressure, exerted by their
elastic walls ; while that of the veins is under little
pressure.

(2.) That the walls of the arteries are strong and re-
sisting, while those of the veins are weak and flabby.

(3.) That the veins have valves opening towards the
heart ; and that, during the diastole, there is no resistance
of any moment to the free passage of blood into the heart ;
while, on the other hand, the cavity of the arteries is shut
off from that of the ventricle, during the diastole, by the
closure of the semilunar valves.

Hence it follows that equal pressures applied to the
surface of the veins and to that of the arteries must

' " A poun 1 " is stated here for simplicity's sake. As a matter of fact the
pressure required is less than this.

IV.] EFFECTS ON THE CIRCULATION. loi

produce very different effects. In the veins the pressure is
something which did not exist before ; and partly from
the presence of valves, partly from the absence of resis-
tance in the heart, partly from the presence of resistance
in the capillaries, it all tends to accelerate the flow of blood
towards the heart. In the arteries, on the other hand, the
pressure is only a fractional addition to that which existed
before ; so that, during the systole, it only makes a com-
paratively small addition to the resistance which has to
be overcome by the ventricle ; and during the diastole, it
superadds itself to the elasticity of the arterial walls in
driving the blood onwards towards the capillaries, inas-
much as all progress in the opposite direction is stopped
by the semilunar valves.

It is, therefore, clear, that the inspiratory movement, on
the whole, helps the heart, inasmuch as its general result
is to drive the blood the way that the heart propels it,

28. In expiration, the difference between the pressure of
the atmosphere on the surface, and that which it exerts
on the contents of the thorax through the lungs, becomes
less and less in proportion to the completeness of the ex-
piration. Whenever, by the ascent of the diaphragm and
the descent of the ribs, the cavity of the thorax is so far
diminished that pressure is exerted on the great vessels,
the veins, owing to the thinness of their walls, are especi-
ally affected, and a check is given to the flow of blood
in them, which may become visible as a venous piclse in
the great vessels of the neck. In its effect on the arterial
trunks, expiration, like inspiration, is, on the whole, favour-
able to the circulation ; the increased resistance to the
opening of the valves during the ventricular systole being
more than balanced by the advantage gained in the addi-
tion of the expiratory pressure to the elastic reaction of
the arterial walls during the diastole.

When the skull of a living animal is laid open and the
brain exposed, the cerebral substance is seen to rise and
fall synchronously with the respiratory movements ; the
rise corresponding with expiration, and being caused by
the obstruction thereby offered to the flow of the blood in
the veins of the head and neck.

29. The activity of the respiratory process is greatly
modified by the circumstances in which the body is placed.

I02 ELEMENTARY PHYSIOLOGY. [less.

Thus, cold greatly increases the quantity of air which is
breathed, the quantity of oxygen absorbed, and of carbonic
acid expelled : exercise and the taking of food have a cor-
responding effect.

In proportion to the weight of the body, the activity of
the respiratory process is far greatest in children, and
diminishes gradually with age.

The excretion of carbonic acid is greatest during the
day, and gradually sinks at night, attaining its minimum
about midnight, or a little after.

Indeed it would appear that the rule that the quantity
of oxygen taken in by respiration is, approximately, equal
to that given out by expiration, only holds good for the
total result of twenty-four hours' respiration. Much more
oxygen appears tq be given out during the day-time (in
combination with carbon as carbonic acid) than is ab-
sorbed ; while, at night, much more oxygen is absorbed
than is excreted as carbonic acid during the same period.
And it is very probable that the deficiency of oxygen
towards the end of the waking hours, which is thus
produced, is one cause of the sense of fatigue which comes
on at that time. This difference between day and night
is, however, not constant, and appears to depend a good
deal on the time when food is taken.

The quantity of oxygen which disappears in proportion
to the carbonic acid given out, is greatest in carnivorous,
least in herbivorous animals — greater in a man living on
a flesh diet, than when the same man is feeding on vege-
table matters.

30. When a man is strangled, drowned, or choked, or
is, in any other way, prevented from inspiring or expiring
sufficiently pijre atmospheric air, what is called asphyxia^
comes on. He grows "black in the face ; " the veins be-
come turgid ; insensibility, not unfrequcntly accompanied
by convulsive movements, sets in, and he is dead in a few
minutes.

It is not necessary, however, violently to strangle,
or drown, a man, in order to asphyxiate him. As, other
things being alike, the rapidity of diffusion between two
gaseous mixtures depends on the difference of the pro-
portions in which their constitutents are mixed, it follows
that the more nearly the composition of the tidal air

IV.] ASPHYXIA. 103

approaches that of the stationary air, the slower will be
the diffusion of oxygen inwards, and of carbonic acid
outwards, and the more defective in oxygen and charged
with carbonic acid will the air in the air-cells become.
Hence even with gradual changes in the air breathed, the
oxygen in the tidal air being gradually diminished and the
carbonic acid in the tidal air being gradually increased, a
point will at length be reached when the change effected
in the stationary air is too slight to enable it to relieve
the pulmonary blood of its carbonic acid, and to supply it
with oxygen to the extent required for its arterialisation.

31. Thus, in all cases of asphyxia however produced,
the blood passing along the pulmonary veins into the left
auricle, instead of being arterial is venous, and becomes
more and more venous at each moment. Hence the
blood distributed by the left ventricle throughout the body
is no longer arterial but venous ; all the tissues and
organs of the body are supplied with venous instead of
arterial blood, and in consequence they all suffer. The
respiratory centre in the medulla (see § 24) is perhaps
the first to feel it ; this gives out impulses which at first
manifest themselves in the form of violent laboured
inspiratory and expiratory efforts, but eventually end in
general convulsions. The brain feels it, and being poisoned
by the venous blood ceases to act, so that consciousness
disappears and insensibility ensues. The heart and blood
vessels feel it and the circulation is disturbed, so that the
heart especially on the right side and the whole venous
system becomes gorged with blood ; hence the blackness
in the face. Eventually the nervous system becomes
exhausted and all the movements of respiration as well
as those of the body at large come to an end ; and
the heart too, poisoned by the continued venous blood,
ceases to beat. Thus death is brought about ; all the
functions of the body are brought to an end because
everywhere there is venous instead of arterial blood.

32. But venous blood is distinguished from arterial by
two features, by having less oxygen and more carbonic
acid. Hence, in this asphyxiating process, two influences
of a distinct nature are co-operating ; one is the deprivation
ofoxygeft^iYiQ. other istho. excessii/e acctanulatioiiofcarbo7tic
acid in the blood. Oxygen starvation and carbonic acid

I04 ELEMENTARY PHYSIOLOGY. [less.

poisoning, each of which is injurious in itself, are at work
together.

The effects of oxygen starvation may be studied sepa-
rately, by placing a small animal under the receiver of an
air-pump and exhausting the air ; or by replacing the
air by a stream of hydrogen or nitrogen gas. In these
cases no accumulation of carbonic acid is permitted, but,
on the other hand, the supply of oxygen soon becomes
insufficient, and the animal quickly dies with all the
symptoms of asphyxia. And if the experiment be made
in another way, by placing a small mammal, or bird, in
air from which the carbonic acid is removed as soon as it
is formed, the animal will nevertheless die asphyxiated as
soon as the amount of oxygen is reduced to lo per cent
or thereabouts.

The directly poisonous effect of carbonic acid, on
the other hand, has been ver}- much exaggerated. A
very large quantity of pure carbonic acid (lo to 15 or
20 per cent.) may be contained in air, without producing
any very serious immediate effect, if the quantity of
oxygen be simultaneously increased.

Moreover such symptoms as do occur when the carbonic
acid in the air breathed is increased without any corre-
sponding decrease in the oxygen, are not exactly those of
asphyxia but are said to resemble rather those of nar-
cotic poisoning. So that the chief cause of asphyxia in
strangling, drowning, or choking, or however produced, is
the diminution of the oxygen in the air of the lungs and
consequently a diminution of the oxygen in the blood.

33. And that it is the lack of oxygen which is the
important thing is further shown by the asph}-xiating
effects of certain poisonous gases. Thus sulphuretted
hydrogen, so well known by its offensive smell, has long
had the repute of being a positive poison. But its evil
effects appear to arise chiefly, if not wholly, from the
circumstance that its hydrogen combines with the oxygen
carried by the blood-corpuscles, and thus gives rise,
indirectly, to a form of oxygen starvation-
Carbonic oxide gas has a much more serious effect, as
it turns out the oxygen from the blood-corpuscles, and
forms a combination of its own with the haemoglobin.
The compound thus formed is only very gradually decom-

IV.] DYSPNCEA. 105

posed by fresh oxygen, so that if any large proportion of
the blood-corpuscles be thus rendered useless, the animal
dies before restoration can be effected. Badly made
common coal gas sometimes contains 20 to 30 per cent, of
carbonic oxide ; and, under these circumstances, a leakage
of the pipes in a house may be extremely perilous to life.

34. The first stages of asphyxia, when the breathing is
simply hurried or violent, before consciousness is lost and
before convulsions set in, is often spoken of as dysp7t(sa^
or laboured breathing. And dyspnoea begins to show
itself as soon as ever there is any serious diminution of the
oxygen in the tidal air. A very slight reduction will hardly
effect the breathing at all or only make it rather quicker
and deeper, but when the proportion of oxygen in the
tidal air is largely diminished, brought down for instance
to 10 per cent., the case becomes serious. And it makes
no difference whether this condition of the tidal air is
brought about by shutting out fresh air, or by augmenting
the number of persons who are consuming the same
air, or by suffering combustion, in any shape, to carry off
oxygen from the air.

But in the case of breathing the same air over and over
again the deprivation of oxygen, and the accumulation of
carbonic acid, cause injury, long before the asphyxiating
point is reached. Under these circumstances uneasiness
and headache arise when less than i per cent, of the
oxygen of the air is replaced by other matters ; the
symptoms in this case however are due not so much to
the diminution of oxygen or the increase of carbonic acid,
as to the poisonous effects of the various organic matters
present in expired air which, though existing in minute
quantities, have a powerfully deleterious action. It need
hardly be added that the persistent breathing of such air
tends to lower all kinds of vital energy, and predisposes
to disease.

Hence the necessity of sufficient air and of ventilation
for every human being. To be supplied with respiratory
air in a fair state of purity, every man ought to have at
least 800 cubic feet of space '^ to himself, and that space
ought to be freely accessible, by direct or indirect chan-
nels, to the atmosphere.

^ A cubical room nine feet high, wide and long, contains only 729 cubic
feet of air.

io6 ELEMENTARY PHYSIOLOGY. [less.

LESSON V.

THE SOURCES OF LOSS AND OF GAIX TO THE BLOOD.

1. The blood which has been aerated, or arteriahsed,
by the process described in the preceding Lesson, is
carried from the lungs by the pulmonary veins to the left
auricle, and is then forced by the auricle into the ven-
tricle, and by the ventricle into the aorta. As that great
vessel traverses the thorax, it gives off several large
arteries, by means of which blood is distributed to the
head, the arms, and the walls of the body. Passing
through the diaphragm (Fig. 23), the aortic trunk enters
the cavity of the abdomen, and becomes what is called
the abdominal aorta, from which vessels are given off to
the viscera of the abdomen. Finally, the main stream
of blood flows into the iliac arteries, whence the viscera
of the pelvis and the legs are supplied.

Having in the various parts of the body traversed the
ultimate ramifications of the arteries, the blood, as we have
seen, enters the capillaries. Here the products of the
waste of the tissues constantly pour into it ; and, as the
blood is everywhere full of corpuscles, which, like all
other living things, decay and die, the products of their
decomposition also tend to accumulate in it, but these are
insignificant compared to those coming from the great
mass of the tissues. It follows that, if the blood is to be
kept pure, the waste matters thus incessantly poured into,
or generated in it, must be as constantly got rid of, or
excreted.

2. Three distinct sets of organs arc especially charged
with this office of continually excreting waste matters
from the blood. They are the lungs, the kidneys, and

v.] LOSSES OF THE BLOOD. 107

the skin (see Lesson L § 23). These three great organs
may therefore be regarded as so many drains from the
blood — as so many channels by which it is constantly
losing substance.

On the other hand, the blood, as it passes through the
capillaries, is constantly giving up material by exudation
through the capillary walls into the surrounding tissues,
in order to supply them with nourishment, and thus in this
way also is constantly losing matter.

The material which the blood loses by giving it up to
the tissues consists of complex organic bodies, such as
proteids, fats, carbohydrates, and various substances manu-
factured out of these, of certain salts, of a large quantity
of water, and lastly of oxygen.

The material which the blood loses by giving it up to
the skin, lungs and kidneys, passes away from these
organs as water, as carbonic acid, as peculiar organic
substances of which one, called tirea^ is much more
abundant than the others, and as certain inorganic salts.
Speaking generally we may say that these organs together
excrete from the blood, water, carbonic acid, urea and
salts.

Another kind of loss takes place from the surface of
the body generally, and from the interior of the air-
passages. Heat is constantly being given off from the
former by radiation, evaporation, and conduction : from
the latter, chiefly by evaporation ; and the loss of heat in
each case is borne by the blood passing through the skin
and air-passages respectively. Besides this a certain
quantity of heat is lost by the urine and faeces which are
always warm when they leave the body.

3. On the side of gain we have, in the first place, the
various substances which are the products of the activity
of the several tissues, muscles, brain, glands, &c., and
which pass from the tissues into the blood. We may
speak of these as waste products, and one of them which is
produced by all the tissues, namely carbonic acid, is em-
phatically a waste product and is got rid of as soon as
possible. But some of the substances which are returned
to the blood from the tissues are not wholly useless matters
to be thrown off as rapidly as possible ; they are capable
of being used up again by some tissue or other. Thus, as

io8 ELEMENTARY PHYSIOLOGY. [less.

we shall see, the liver, at certain times at all events, returns
to the blood a certain quantity of sugar which is made
use of in other parts of the body, and similarly the spleen,
while it takes up certain substances from the blood, gives
back to the blood certain other substances which we can
hardly speak of as waste matters in the sense of being
useless material fit only to be at once thrown away.

In the second place, the blood is continually receiving
from the alimentary canal the materials arising from the
food which has been digested there. As we shall see, some
of this material passes directly from the cavit>' of the
alimentary canal into the blood, but some of it goes in a
more roundabout way through what are called the lacteals
or lymphatics. On its way to the blood this latter is
joined by material which, escaping from the blood and
not used by the tissues, or passing from the tissues directly
into the lymphatics, is carried back to the blood by the
thoracic duct (see Less. II. § 5).

In the third place, the blood is continually gaining oxygen
from the air through the lungs.

Then again the blood while it loses heat by the skin and
lungs, gains heat from the tissues. As we have already
seen (Less. I. § 24) oxidation is continually going on in
various parts of the body, and by this oxidation heat is
continually being set free. Some of this oxidation may
take place in the blood itself ; we do not know exactly how
much, but probably ver}' little. The greater part of the heat
is generated in the tissues, in the muscles and elsewhere,
and is given up by the tissues to the blood. So that we
may say that the blood gains heat from the tissues.

4. These several gains and losses are for the most part
going on constantly but are greater at one time than at
another. Thus the gain to the blood from the alimentary
canal is much greater some time after a meal than just
before the next meal, though unless the meals be very far
apart indeed, the whole of the material of one meal has
not passed into the blood before the next meal is begun.
Again, though the muscles, even when completely at rest,
are taking up oxygen and nutritive material, and giving
out carbonic acid and other waste products, they give out
and take in much more when they are at work. So also
certain " secreting glands " as they are called, which we

v.] GAINS AND LOSSES OF THE BLOOD. 109

shall study presently, such as the salivary glands, have
periods of repose ; it is at certain times only, as when
food has been taken, that they pour out any appreci-
able quantity of fluid. Hence though they are probably
taking up material from the blood and storing it up in
their substance even when they appear at rest, they take
up much more and so become much more distinctly
means of loss to the blood, when they are actively pouring
out their secretions. In the case of the liver the loss to
the blood is more constant, since the secretion of bile as
we shall see is continually going on, though greater at
certain times than at others ; and the materials for the
bile have to be provided by the blood. Some of the
constituents of the bile, however, pass back from the
intestines into the blood ; and so far the loss to the blood
by the liver is temporary^ only.

Of all the gains to the blood perhaps the most con-
stant is that of oxygen, and of all the losses perhaps the
most constant is that of carbonic acid ; but even these
vary a good deal at different times or under different cir-
cumstances.

Broadly speaking then the blood gains oxygen from the
lungs, complex organic food materials from the alimentar}'
canal, and various substances which we may speak of as
waste substances from the several tissues ; and it loses on
the one hand material which we may speak of as con-
structive material to the several tissues, and on the other
hand material which passes away by the skin, lungs,
and kidney, as water, carbonic acid, urea, and saline
bodies.

And while it is continually receiving heat from the
several tissues, it is also continually losing heat by the
skin, lungs, and other free surfaces of the body.

5. The sources of loss and gain to the blood may be
conveniently arranged in the following tabular form : —

no ELEMENTARY PHYSIOLOGY. [less.

Sources of Loss or Gain to the Blood.*

A. Sources of Gain : —

L Ga/n of Matter.

1. The lungs : oxygen (fairly constant).

2. The alimentary canal : food (variable).

3. The tissues : products of their activity, waste

matters (always going on but varying
according to the activity of the several
tissues).

4. The lymphatics : lymph (always going on but

varying according to the activity of the
several tissues). -

IL Cain of Heat.

1. The tissues generally, especially the more

active ones, such as the muscles.

2. The blood itself, probably to a small extent.

B. Sources of Loss :—

L Loss of Matter.

1. The lungs : carbonic acid and water (fairly

constant).

2. The kidneys : urea, water, salines (fairly

constant).

3. The skin : Avater, salines (fairly constant).

4. The tissues : constructive material (variable

especially in the case of those tissues
whose activity is intermittent, such as the
muscles, many secreting glands, &c.).

n. Loss of Heat.

1. The skin.

2. The lungs.

3. The excretions by the kidney and the alimen-

tary canal.

^ The learner must be careful not to confound the losses and gains of the
blood with the losses and gains of the body as a whole. The two differ in
much the same way as the internal commerce of a country differs from its
export and import trade.

2 The gain from those lymphatics which are called lacteal.-;, since it comei
from the alimentary canal, varies much more.

v.]

THE KIDNEYS.

Ill

6. In the preceding Lesson I have described the ope-
ration by which the lungs withdraw from the blood much
carbonic acid and water, and supply oxygen to the blood ;
I now proceed to the second source of continual loss, the
Kidneys.

Of these organs, there are two, placed at the back of the
abdominal cavity, one on each side of the lum.bar region
of the spine. Each, though somewhat larger than the
kidney of a sheep, has a similar shape. The depressed, or
concave, side of the kidney is turned inwards, or towards
the spine ; and its convex side is directed outwards (Fig.
25). From the middle of the concave side (called the
hilus) of each kidney, a long tube with a small bore, the
Ureter {Ur), proceeds to the bladder {Bf).

vx.r

Fig. 25.

The kidneys (-/T) ; ureters {Ur); with the aorta {Ao), and vena cava in-
ferior {V.C.I) : and the renal arteries and veins. B/, is the bladder, the
top of which is cut off so as to show the openings of the ureters (i, r)
and that of the urethra (2).

112 ELEMENTARY PHYSIOLOGY. [less.

The latter, situated in the pelvis, is an oval bag, the
walls of which contain abundant unstriped muscular fibre,
while it is lined, internally, by mucous membrane, and
coated externally by a layer of the peritoneum, or double
bag of serous membrane which has exactly the same rela-
tions to the cavity of abdomen and the viscera contained
in them as the pleurae have to the thoracic cavity and the
lungs. The ureters open side by side, but at some little
distance from one another, on the posterior and inferior
wall of the bladder (Fig. 25, i, i). In front of them is
a single aperture which leads into the canal called the
Urethra (Fig. 25, 2), by which the cavity of the bladder is
placed in communication with the exterior of the body.
The openings of the ureters enter the walls of the bladder
obliquely, so that it is much more easy for the fluid to
pass from the ureters into the bladder than for it to get
the other way, from the bladder into the ureters.

Mechanically speaking, there is little obstacle to the
free flow of fluid from the ureters into the bladder, and
from the bladder into the urethra, and so outwards ; but
certain muscular fibres arranged circularly around the
part called the " neck '' of the bladder, where it joins the
urethra, constitute what is termed a sphincter^ and are
usually, during life, in a state of contraction, so as to
close the exit of the bladder, while the other muscular
fibres of the organ are relaxed.

It is only at intervals that this state of matters is
reversed ; and the walls of the bladder contracting, while
its sphincter relaxes, its contents, the urine, are dis-
charged. But, though the expulsion of the secretion of
the kidneys from the body is thus intermittent, the excre-
tion itself is constant, and the urinar)' fluid flows, drop by
drop, from the opening of the ureters into the bladder.
Here it accumulates, until its quantity is sufficient to
give rise to the uneasy sensations which compel its ex-
pulsion.

7. The renal excretion has naturally an acid reaction,
and consists chiefly of urea with a small quantity of uric
acid, sundry other animal products of less importance,
including certain colouring matters, and saline and gase-
ous substances, all held in solution by a large quantity of
water.

v.] UiaNE. 113

The quantity and composition of the urine vary greatly
according to the time of day ; the temperature and mois-
ture of the air ; the fasting or replete condition of the
alimentary canal ; and the nature of the food.

Urea and uric acid are both composed of the elements
carbon, hydrogen, oxygen, and nitrogen ; but the urea is
by far the more soluble in water, and greatly exceeds the
uric acid in quantity.

An average healthy man excretes by the kidneys about
fifty ounces, or 24 000 grains of water a day. In this are
dissolved 500 grains of urea, but not more than 10 to 12
grains of uric acid.

The amount of other animal matters, and of saline sub-
stances, varies from one-third as much to nearly the same
amount as the urea. The saline matters consist chiefly of
common salt, phosphates and sulphates of potash, soda,
lime, and magnesia. The gas which is dissolved in the
urine consists chiefly of carbonic acid, with a very small
quantity of nitrogen and still less of oxygen.

The average specific gravity does not differ very widely
from that of blood serum, being i •020.

8. The excretion of nitrogenous waste and water, with
a little carbonic acid, by the kidneys, is thus strictly com-
parable to that of carbonic acid and water, by the lungs,
in the air-cells of which carbonic acid and watery vapours
are incessantly accumulating, to be periodically expelled
by the act of expiration. But the operation of the renal
apparatus differs from that of the respiratory organs in the
far longer intervals between the expulsory acts ; and still
more in the circumstance that, while the substance which
the lungs take into the body is as important as those
which they give out, the kidneys take in nothing.

9. We have reason to think that many of the con-
stituents of the urine are present in the blood. These
appear in the urine dissolved in a large quantity of water,
whereas many other substances also present in the blood
do not, in a state of health, make their way into the
urine. This suggests the idea that the kidney is a peculiar
and delicate kind of filter which allows certain substances
together with a large quantity of water to pass through it,
but refuses to allow other substances to pass through.
And when we come to study the minute structure of the

ELEMENTARY PHYSIOLOGY.

[less.

kidney, to which we must now turn, we find much to
support this idea.

When a longitudinal section of a kidney is made (Fig.
26), the upper end of the ureter {U) seems to widen out
into a basin-like cavity {P), which is called iht pelvis of
the kidney. Into this, sundry conical elevations, called the
pyramids {Py) project ; and their summits present multi-
tudes of minute openings — the final terminations of thft

M-

Fig. 26.— Longitudinal Section of the Human Kidney.

Ct, the cortical substance ; M, the medullary substance ; P, the pelvis of
the kidney ; U, the ureter; RA, the renal artery ; Py, the pyramids.

tubuli, of which the thickness of the kidney is chiefly made
up. If the tubules be traced from their openings towards
the outer surface, they are found, at first, to lie parallel
with one another in bundles, which radiate towards the
surface, and subdivide as they go ; but at length they
spread about irregularly, and become coiled and interlaced.
From this circumstance, the middle, or viedullary, part
{^medulla, marrow) of the kidney looks different from the

v.]

THE KIDNEY.

115

superficial, or cortical^ part {cortex^ bark) ; but, in addition,
the cortical part is more abundantly supplied with vessels
than the medullary, and hence has a darker aspect. Each
tubule after a very devious course ultimately terminates
in a dilatation (Fig. 28) called a Malpighian capsule. Into
the summit of each capsule, a small vessel (Figs. 28 and
29, v.a)^ one of the ultimate branches of the renal artery^
which reaches the kidney at the concave side, with the

Fig. 27. — Diagrammatic View of the Course of the Tubules
IN the Kidney.

r, cortical portion answering to C/ in Fig 26, k being close to the surface

of tile kidneys; g, p, medullary portion, / reaching to the summit of

the pyramid.
IX, opening of tubule on the pyramid ; VIII, VII, VI, the straight

portion of the tubules ; V—II, the twisted portion of the tubules ; /,

the Malpighian capsule.

ureters, and divides into branches which pass in between
the pyramids (Fig. 26, RA), enters (driving the thin wall
of the capsule before it), and immediately breaks up into

I 2

Il6

ELEMENTARY PHYSICyLOGY.

[less.

Fig. 28. — A Malpighian Capsule (highly magnified).

va, small branch of renal artery entering the capsule, breaking up into the
glomerulus, g.l, and finally joining again to form the vein, v.c.

c, the tubule ; a, the epithelium over the glomerulus ; i>, the epithelium
lining the capsule.

Fig. 29. — Circulation in the Kidney.

ai, small branch of renal artery giving off the branch 7'a, which enters
glomerulus, issues as 7'e, and then breaks up into capillaries, which after
surrounding the tubule find their way by v into vi, branch of the renal
vein ; w/, capillaries around tubules in parts of the cortical substance where
there are no glomeruli.

v.] THE KIDNEY. 117

a bunch of looped capillaries, called a glomerulus (Fig.
28, ,ir-^) which nearly fills the cavity of the capsule The
blood is carried away from this glomerulus by a small
vein or vessel {v.e), which does not, at once, join with
other veins into a larger venous trunk, but opens into
the network of capillaries (Fig. 29) which surrounds the
tubule, thus repeating the portal circulation on a small
scale.

The tubule has an epithelial lining (Fig. 28, c, and Fig.
30, a), continuous with that of the pelvis of the kidney
and the urinary passages generally. The epithelium is
thick and plain enough in the tubule, but it becomes very
delicate in the capsule and on the glomerulus (Fig. 28,
a. b).

10. It is obvious from this description, that the surface
of the glomerulus is, practically, free, or in direct commu-
nication with the exterior by
means of the cavity of the
tubule ; and further, that, in
each vessel of the glomerulus,
a thin stream of blood constantly
flows, only separated from the
cavity of the tubule by the
capillary wall and the very deli-
cate membrane covering the _________

glomerulus. ^ The Malpighian ^^^ jo.-Tkansverse Section"
capsule may, m fact, be regarded of two Tubules.

as a funnel, and the membranous a.a, canals of tubules surrounded

walls of the glomerulus as a , ^y^}^'""^ epithelium.

f. T T ^ 1 i i^j a blood-vessel cut across.

piece of very delicate but pecu-
liar filtering-paper, into which the blood is poured.

11. And indeed we have reason to think that a great
deal of the water of urine together with certain of the
constituents is thus as it were filtered by the Malpighian
capsules. But it must be remembered that the process is
after all very different from actual filtering through blotting
paper ; for blotting paper will let everything pass through
that is really dissolved, whereas the glomerulus, while
letting some things through, will refuse to admit others
even though completely dissolved.

Speaking of the process, with this caution, as one of
filtration, it is obvious that the more full the glomerulus is

lis ELEMENTARY PHYSIOLOGY. [less.

of blood the more rapid will be the escape of urine.
Hence we find that when blood flows freely to the kidney
the urine is secreted freely, but that when the blood
supply to the kidney is scanty the urine also is scanty.
When certain nerves going to the kidney are cut, the
ramifications of the renal artery dilate, much blood goes
into the kidney and the flow of urine is copious. If the
same nerves be irritated, the arterial tubes are narrowed or
constricted, less blood goes to the kidney, and the flow of
urine is scanty or may be stopped altogether.

And this explains, in part at all events, how it is that
the activity of the kidney is influenced by the state of the
skin. The quantity of blood in the body, being about the
same at all times, if a large quantity goes to the skin, as
in warm weather and especially when the skin is active
and perspiring, less will go to the kidney, and the secretion
of urine will be small. On the other hand, if the blood
be largely cut off from the skin, as in cold weather, more
blood will be thrown upon the kidney and more urine
will be secreted. Thus the skin and the kidneys play
into each other's hands in their efforts to get rid of the
superfluous water of the body.

12. But the whole of the urine is thus not secreted,
through a sort of filtering process, by the Malpighian
capsules. The tubules are lined, as has been stated,
by epithelium cells, and these cells, in certain parts of the
tubule, especially where these are coiled, are what is called
secreting' cells. That is to say they have the power, by
some means which we do not at present fully understand,
to take up from the blood, which is flowing in the capilla-
ries wound round the tubules, or rather from the plasma
which exudes from those capillaries and bathes the bases
of the cells, certain substances, and to pour these sub-
stances, in some cases greatly changed, in some cases
hardly or not at all changed, into the cavity of the
tubule. As has been said, even the blood which escapes
from the glomerulus and has therefore parted with some
of the substances which go to form the urine, is carried
to the capillary network wrapped round the tubules, and
is there exposed to the further action of the epithelium
cells which line those tubules, the plasma which exudes
from the capillaries acting as a middle man between the

v.] PERSPIRATION. 119

blood inside the capillary walls and the substance of the
cells themselves.

And we have evidence that many of the most important
constituents of the urine, such as urea, uric acid and
others, are thus secreted by the epithelium cells of the
tubules, and not simply filtered oft' by the Alalpighian
capsules.

The formation of urine is therefore a double process.
A great deal of the water, with probably some of the
more soluble inorganic salts, pass by the glomeruli, but
the urea, the colouring matters and a great many other of
the constituents, are thrown into the cavities of the tubules
by a peculiar action of the epithelium cells, some of those
substances being actually manufactured by the cell and
not existing as such in the blood.

13. That the skin is a source of continual loss to the
blood may be proved in various ways. If the whole body
of a man, or one of his limbs, be enclosed in a caoutchouc
bag, full of air, it will be found that this air undergoes
changes which are similar in kind to those which take
place in the air which is inspired into the lungs. That is
to say, the air loses oxygen and gains carbonic acid ; it
also receives a great quantity of watery vapour, which
condenses upon the sides of the bag, and may be drawn
off by a properly disposed pipe.

Under ordinary circumstancei no liquid water appears
upon the surface of the integument, and the whole process
receives the name of the insensible perspiratioji. But,
when violent exercise is taken, or under some kinds of
mental emotion, or when the body is exposed to a hot and
moist atmosphere, the perspiration becomes sensible;
that is, appears in the form of scattered drops upon the
surface.

14. The quantity o{ siucat^ or sensible perspiration, and
also the total amount of both sensible and insensible per-
spiration, vary immensely, according to the temperature
and other conditions of the air, and according to the state
of the blood and of the nervous system. It is estimated
that, as a general rule, the quantity of water excreted by
the skin is about double that given out by the lungs in the
same time. The quantity of carbonic acid is not above
Tijth or iVth of that excreted by the lungs ; and it is not

I20 ELEMENTARY PHYSIOLOGY. [less.

certain that in health any appreciable quantity of urea is
given off.

In its normal state the sweat, as poured out from the
proper sweat-glands, is alkaline ; but ordinarily, as it col-
lects upon the skin it is mixed with the fatty secretion of
the sebaceous glands, and then is frequently acid. In
addition it contains scales of the external layers of the
epidermis, which are constantly being shed.

15. In analysing the process by which the perspiration
is eliminated from the body, it must be recollected, in the
first place, that the skin, even if there were no glandular
structures connected with it, would be in the position of a
moderately thick, permeable membrane, interposed be-
tween a hot fluid, the blood, and the atmosphere. Even
in hot climates the air is, usually, far from being com-
pletely saturated witli watery vapour, and in temperate
climates it ceases to be so saturated the moment it comes
into contact with the skin, the temperature of which is,
ordinarily, twenty or thirty degrees above its own.

A bladder exhibits no sensible pores ; but if a bladder
be filled with water and suspended in the air, the water
will gradually ooze through the walls of the bladder, and
disappear by evaporation. Now, in its relation to the
blood, the skin is such a bladder full of hot fluid.

Thus, perspiration, to a certain amount, must always be
going on through the substance of the integument, but
probably not to any great extent ; though what the amount
of this perspiration may be cannot be accurately ascer-
tained, because a second and very important source of
the perspiration is to be found in what are called the
sweat-glands.

16. All over the body the integument presents minute
apertures, the ends of channels excavated in the epidermis
or scarf-skin, and each continuing the direction of a
minute tube, usually about Triijth of an inch in diameter,
and a quarter of an inch long, which is imbedded in the
dermis. Each tube is lined with an epithelium continu-
ous with the epidermis (Fig. 32, e). The tube sometimes
divides, but, whether single or branched, its inner end or
ends are blind, and coiled up into a sort of knot, inter-
laced with a meshwork of capillaries (Fig. 31, Kg, and
Fig. 33)-

v.]

SWEAT-GLANDS.

121

The blood in these capillaries is therefore separated
from the cavity of the sweat-gland only by the thin walls
of the capillaries, that of the glandular tube, and its
epithelium, which, taken together, constitute but a very
thin pellicle ; and the arrangement, though different in
detail, is similar in principle to that which obtains in the
kidney. In the latter, the vessel makes a coil within the
Malpighian capsule, which ends a tubule. Here the
perspiratory tubule coils about, and among, the vessels.
In both cases the same result is arrived at — namely, the

Fig. 31.

A. Section of the skin showing the sweat-glands, a, the epidermis ; i5, its
deeper layer, the reie Malpighii ; e,d, the dermis or trueskm ;/, fat cells ;
g, the coiled end of a sweat-gland ; h, its duct ; /, its openmg on the surtace
of the epidermis. , , , . , , v

B. Section of the skin sho\ving the roots of the hairs and the sebaceous
glands, b, muscle of c, the hair sheath, on the left hand.

exposure of the blood to a large, relatively free, surface,
on to which certain of its contents transude. In the
sweat-gland however there is no filtering apparatus like
the Malpighian corpuscle of the kidney, and the whole
of the sweat appears to be secreted into the interior of the
tube by the action of the epithelium cells which line it.
The number of these glands varies in different parts of

122

ELEMENTARY PHYSIOLOGY,

[less.

X — a

Fig. 32.

Portion of Fig. 31 A more highly magnified— somewhat diagrammatic, a
horny epidermis ; b softer layer, rcte Malpighli ; c, dermis ; d, lowermost
vertical layerof epidermic cells ; ^, cells lining the sweat duct continuous
with epidermic cells ; //, corkscrew canal of sweat duct. To the right of the
sweat duct the dermis is raised into a papilla, in which the small arter\- X
breaks up into capillaries, ultimately forming the veins g

v.]

SWEAT-GLANDS.

123

the body. They are fewest in the back and neck, where
their number is not much more than 400 to a square inch.
They are more numerous on the skin of the palm and
sole, where their apertures follow the ridges visible on the
skin, and amount to between two and three thousand on
the square inch. At a rough estimate, the whole integu-
ment probably possesses not fewer than from two millions
and a quarter to two millions and a half of these tubules,
which therefore must possess a very great aggregate
secreting power.

Fig. 33-

Coiled end of a sweat-gland (Fig. 31. ^), epithelium uot shown. « the coil ;

b, the duct ; c, network of capillaries, inside which tha duct gland hes.

17. The sweat-glands are greatly under the influence of
the nervous system. This is proved, not merely by the
well-known effects of mental emotion in sometimes sup-
pressing the perspiration and sometimes causing it to be
poured forth in immense abundance, but has been made a
matter of direct experiment. There are some animals,
such as the horse, which perspire very freely. If the

124 ELEMENTARY PHYSIOLOGY. [less.

sympathetic nerve of one side, in the neck of a horse, be
cut, the same side of the head becomes injected with
blood, and its temperature rises (see Lesson ii. § 24) ;
and, simultaneously, sweat is poured out abundantly over
the whole surface thus affected. On irritating that end of
the cut nerve which is in connection with the vessels, the
muscular walls of the latter, to which the nerve is distri-
buted, contract, the congestion ceases, and with it the
perspiration.

On the other hand, experiments have been made on
other nerves in other animals in which it is seen that
section of the nerve stops perspiration, while stimulation
of it causes perspiration, and that independently of any
changes in the condition of the blood-vessels. Such
nerves may be called ' sweat-nerves ' inasmuch as stimu-
lation of them directly excites perspiration.

18. The amount of matter which may be lost by per-
spiration, under certain circumstances, is very remarkable.
Heat and severe labour, combined, may reduce the weight
of a man two or three pounds in an hour, by means of the
cutaneous perspiration alone ; and, as there is some reason
to believe that the quantity of sol id matter carried off from
the blood does not diminish with the increase of the amount
of the perspiration, the total amount of solids which are
eliminated by profuse sweating may be considerable.

The difference between blood which is coming from,
and that which is going to, the skin, can only be con-
cluded from the nature of the substances given out in the
perspiration ; but arterial blood is not rendered venous
in the skin.

19. It will now be instructive to compare together in
more detail than has been done in the first Lesson (§ 23),
the three great organs — lungs, kidneys, and skin — which
have been described.

In ultimate anatomical analysis, each of these organs
consists of a moist animal membrane separating the blood
from the atmosphere.

Water, carbonic acid, and solid matter pass out from
the blood through the animal membrane in each organ,
and constitute its secretion or excretion ; but the three
organs differ in the absolute and relative amounts of the
constituents the escape of which they permit.

v.] THE LIVER. 125

Taken by weight, water is the predominant excretion in
all three ; most solid matter is given off by the kidneys ;
most gaseous matter by the lungs.

The skin partakes of the nature of both lungs ana
kidneys, seeing that it absorbs oxygen and exhales car-
bonic acid and water, like the former, while it excretes
organic and saline matter in solution, like the latter ; but
the skin is more closely related to the kidneys than to the
lungs. Hence, as has been already said, when the free
action of the skin is interrupted, its work is usually thrown
upon the kidneys, and vice versa. In hot weather, when
the excretion by the skin increases, that of the kidneys
diminishes, and the reverse is observed in cold weather.

This power of mutual substitution, however, only goes
a little way ; for if the kidneys be extirpated, or their
functions much interfered with, death ensues, however
active the skin may be. And, on the other hand, if the
skin be covered with an impenetrable varnish, the tempe-
rature of the body rapidly falls, and death takes place,
though the lungs and kidneys remain active.

20. The liver is a constant source both of loss, and, in
a sense, of gain, to the blood which passes through it. It
gives rise to loss, because it secretes a peculiar fluid, the
bile^ from the blood, and throws that fluid into the intes-
tine. It is also in another way a source of loss because
it elaborates from the blood passing through it a substance
Z2S\.^^ glycogeji, which is stored up sometimes in large,
sometimes in small, quantities in the cells of the liver.
This latter loss, however, is only temporary, and may be
sooner or later converted into a gain, for this glycogen
very readily passes into sugar, and either in that form or
in some other way is carried off by the blood. In this
respect, therefore, there is a gain to the blood of kind or
quality though not of quantity of material.

The liver is the largest glandular organ in the body,
ordinarily weighing about fifty or sixty ounces. It is a
broad, dark, red-coloured organ, which lies on the right
side of the body, immediately below the diaphragm, with
which its upper surface is in contact, while its lower sur-
face touches the intestines and the right kidney.

The liver is invested by a coat of peritoneum, which
keeps it in place. It is flattened from above downwards

126

ELEMENTARY PHYSIOLOGY.

[less.

and convex and smooth above, where it fits into the con-
cavity of the lower surface of the diaphragm. Flat and
irregular below (Fig. 34), it is thick behind, but ends in a
thin edge in front.

Viewed from below, as in Fig. 34, the inferior ve?ia cava,
a, is seen to traverse a notch in the hinder edge of the
liver as it passes from the abdomen to the thorax. At b
the trunk of the ve7ia portcE is observed dividing into the
chief branches which enter into, and ramify through, the
substance of the organ. At d, the hepatic artery, coming
almost directly from the aorta, similarly divides, enters
the liver, and ramifies through it. At c is the single

Fig. 34.— The Liver Turned Up and Viewed from Below.

rt, vena cava ; h,_ vena portae ; c, bile duct ; d, hepatic artery ; /, gall-bladder.
The termination of the hepatic vein in the vena cava is not seen, being
covered by the piece of the vena cava.

trunk of the duct, called the hepatic duct, which conveys
away the bile brought to it by its right and left branches
from the liver. Opening into the hepatic duct is seen the
duct of a large oval sac, /, the gall-bladder. The duct is
smaller than the artery, and the artery than the portal vein.
If the branches of the artery, the portal vein, and the
bile duct be traced into the substance of the liver, they
will be found to accompany one another, and to branch
out and subdivide, becoming smaller and smaller. At
length the portal vein and hepatic artery (Fig. 37, V.P.)
will be found to end in the capillaries, which traverse, like

v.]

THE LIVER.

127

a network, the substance of the smallest subdivisions of the
liver substance visible to the naked eye — polygonal masses
of one-tenth of an inch in diameter, or less, which are
termed the lobules. Every lobule is seated by its base upon
one of the ramifications of a great vein — the hepatic vein —
and the blood of the capillaries of the lobule is poured
into that vein by a minute veinlet, called intralobular

Fig- 35
A Section of part of the Liver to show H.V., a branch of the hepatic vein,
with Z., the lobules or acini of the liver, seated upon its walls, and send-
ing their intralobular veins into it.

(Fig. 37, H. v.), which traverses the centre of the lobule,
and pierces its base. Thus the venous blood of the portal
vein and the arterial blood of the hepatic arter\' reach the
surfaces of the lobules by the ultimate ramifications of
that vein and arter\-, become mixed in the capillaries of
each lobule, and are carried off by its intralobular vqiuIqI^

128

ELEMENTARY PHYSIOLOGY.

[less.

which pours its contents into one of the ramifications of
the hepatic vein. These ramifications, joining together,
form larger and larger trunks, which at length reach the
hinder margin of the liver, and finally open into the vena
cava inferior^ where it passes upwards in contact with
that part of the organ.

Thus the blood with which the liver is supplied is a
mixture of arterial and venous blood : the former brought
by the hepatic artery directly from the aorta, the latter
by the portal vein from the capillaries of the stomach,
intestines, pancreas, and spleen.

Fig. 36.

Termination of bile duct at edge of lobule (somewhat diagrammatic).

^, small bile duct, becoming still smaller at b' , the low flat epithelium at last
suddenly changing into the hepatic cells, /, the channel of the bile duct
being continued as small passages between the latter, c, capillary blood-
vessel cut across.

In the lobules themselves all the meshes of the blood-
vessels are occupied by the liver cells^ or hepatic cells.
These are many-sided minute bodies, each about ruVirth
of an inch in diameter, possessing a nucleus in its inte-
rior, and frequently having larger and smaller granules of
fatty matter distributed through its substance (Fig. 37, rt).
It is in the liver cells that the active powers of the liver
reside.

v.] BILE. 129

The smaller branches of the hepatic duct, lined by an
epithelium, which is continuous with that of the main
duct, and thence with that of the intestines, into which
the main duct opens, may be traced to the very surface of
the lobules, where they seem to end abruptly. But, upon
closer examination, it is found that they communicate
with a network of minute passages passing between the
hepatic cells, and traversing the lobule in the intervals
left by the capillaries Fig. 37, B;. The bile manufactured
by the hepatic cells finds its way first into these minute
passages, and from them into the ducts.

21. The work of the liver, and this, as has been said, is
carried out by the hepatic cells, may be considered as
consisting of two kinds.

On the one hand, the hepatic cells are continually en-
gaged in the manufacture of a complex fluid called bile,
which they pour into the minute passages spoken of
above, and thence into the branches of the hepatic duct ;
whence it flows through the duct itself into the intestines,
or, when digestion is not going on and the opening of the
duct into the intestine is closed, back to the gall-bladder.
The materials for this bile are supplied to the hepatic
cells by the blood : hence the secretion of the bile consti-
tutes a loss to the blood.

22. The total quantity of bile secreted in the twenty-
four hours varies, but probably amounts to not less than
from two to three pounds. It is a golden yellow, slightly
alkaline fluid, of extremely bitter taste, consisting of water
with from 17 per cent, tohalf that quantity of solid matter
in solution. The solids consist in the first place of a
somewhat complex substance which may be separated
out by crystallisation, as an apparently simple mass, but
is in reality a mixture of two acids, in combination with
soda ; one called glycocholic^ and consisting of carbon,
hydrogen, nitrogen and oxygen, the other taurocholic,zxi^
containing, in addition to the other elements, a consider-
able quantity of sulphur. Besides the taurocholate and
glycocholate of soda, or bile salts as the two are sometimes
called, the bile contains a remarkable cr\-stalline sub-
stance, ver\- fatty- looking, but not really of a fatty nature,
called cholesterin, one or more peculiar colouring matters

I30

ELEMENTARY PHYSIOLOGY. [less.

Fig. 37.

A. Section of partially injected liver magnified. The artificial white line

is introduced to mark the limits of a lobule. ^-^^ branches of portal

vein breaking up into capillaries, which run towards the cenUe of the

lobule and join //./•., the intralobular branch of the hepaUc vein. The

v.] BILE. 131

probably related to the ha^matiii of the blood, and certain
saline matters.

23. Of these constituents of the bile the essential sub-
stances, the bile acids and the colouring matter, are not
discoverable in blood which enters the liver ; they must
therefore be formed in the hepatic cells. How they are
exactly formed we do not at present clearly know. The
material of which they are composed is brought to the
hepatic cells by the blood, but the exact condition of
that material — whether, for instance, the blood brings
something very like the bile acids, and only needing a
slight change to be converted into bile acids ; or whether
the hepatic cells manufacture the bile acids from the be-
ginning, as it were, out of the common material which
the blood brings to the liver as to all other tissues and
organs — is not as yet quite determined. The saline matters
and cholesterin, on the other hand, appear to be present
in the blood of the portal vein, and may therefore, like
the water, be simply taken up by the cells from the blood,
and passed on to the bile ducts.

24. Thus the bile is a continual loss to the blood. But,
besides forming bile, the hepatic cells are concerned in
other labours, the result of which can hardly be con-
sidered either as a loss or as a gain, since these labours
simply consist in manufacturing from the blood and stor-
ing up in the hepatic cells substances which, sooner or
later, are returned, generally in a changed condition,
back into the blood.

As we shall presently see, the portal blood is, after a
meal, heavily laden with substances, the result of the
digestive changes in the alimentary canal. When these
substances, carried along in the portal blood, reach the
hepatic cells, in the meshes of the lobules, some of them
appear to be taken up by those cells and to be stored up
in them in a changed condition. In fact, the products of
digestion passing along the portal veins suffer (in the
liver) a further change, which has been called a secondary

outline of the liver cells are seen as a fine network of lines throughout the
whole lobule.
B, Portion of lobule very highly magnified, a, liver cell with n, nucleus (two
are often present) ; b, capillaries cut across ; c, minute biliary passages
between the cells, injected with colouring matter.

K 2

132 ELEMENTARY PHYSIOLOGY. [less.

digestion. Thus the hver produces a powerful effect on
the quahty of the blood passing through it, so that the
blood in the hepatic vein is very different, especially after
a meal, from the blood in the portal vein.

The changes thus effected by the hepatic cells are
probably very numerous, but they have not been fully
worked out, except in one particular case, which is very
interesting and deserves special attention.

It is found that the liver of an animal which has
been well and regularly fed, when examined immediately
after death, contains a considerable quantity of a sub-
stance which is very closely allied to starch, consisting
of carbon, hydrogen, and oxygen in certain proportions.
This substance, which may by proper methods be ex-
tracted and preserved as a white powder, is in fact an
animal starchy and is called ^^p'/y^^^^;^. As we shall see,
common starch is readily changed by certain agents into
grape-sugar, dextrose or glucose, as it is sometimes called ;
and this glycogen is similarly converted with ease into
grape-sugar. Indeed, if the liver of such an animal as
the above, instead of being examined immediately after
death, be left in the body, or be placed on one side after
removal from the body for some hours before it is exa-
mined, a great deal of the glycogen will have disappeared,
a quantity of grape-sugar having taken its place. There
seems to be present in the liver some agent capable of
converting the glycogen into grape-sugar, and this change
is particularly apt to take place if the liver is kept at
blood-heat or near that temperature.

Now if, instead of the liver of a well-fed animal, the
liver of an animal which has been starved for several days
be examined in the same way, very little glycogen indeed
will be found in it, and when the liver is left exposed to
warmth for some time very little grape-sugar is found.
That is to say, the liver has, in the first case, formed the
glycogen and stored it up in itself, out of the food brought
to it by the portal blood : in the second case, no food has
been brought to the liver from the alimentary canal, no gly-
cogen has been formed, and none stored up. If the liver
in the first case be examined microscopically with certain
precautions, the glycogen may be seen stored up in the
hepatic cells ; in the second case little or none can be seen.

v.] GLYCOGEN. 133

The kind of food which best promotes the storing up
of glycogen in the Hver is one containing starch or sugar ;
but some glycogen will make its appearance even when
an animal is fed on an exclusively proteid diet, though not
nearly so much as when starch or sugar is given.

It would appear, then, that the hepatic cells can manu-
facture and store up in themselves the substance glycogen,
being able to make it out of even proteid matter, but more
easily making it out of sugar ; for, as we shall see, all the
starch which is eaten as food is converted into sugar in
the alimentary canal, and reaches the liver as sugar.

There are reasons for thinking that the glycogen, thus
deposited and stored up in the liver, is converted into
sugar little by little as it is wanted, poured into the
hepatic vein, and thus distributed over the body. So that
we may regard this remarkable formation of glycogen in
the liver as an act by which the blood, when it is over-
rich in sugar, as after a meal, stores it up or deposits it in
the liver as glycogen ; and then, in the inter^-als between
meals, the liver deals out the stored-up material as sugar
back again in driblets to the blood. The loss to the blood,
therefore, is temporary — no more a real loss than when a
man deposits at his bankers some money which he has
received until he has need to spend it.

This story of glycogen, important in itself, is also use-
ful as indicating other possible effects of a similar nature
which the hepatic cells may bring about on the blood, as
it is passing in the meshes of the lobules of the liver from
the veinlets of the portal to the veinlets of the hepatic vein.

25. We must next consider the chief sources of con-
stant gain to the blood ; and, in the first place, the sources
Oil gain of matter.

The lungs and skin are, as has been seen, two of the
principal channels by which the body loses liquid and
gaseous matter, but they are also the sole means by which
one of the most important of all substances for the main-
tenance of life, oxygen, is introduced into the blood. It
has already been pointed out that the volume of the
oxygen taken into the blood by the lungs is rather
greater than that of the carbonic acid given out. The
absolute weight of oxygen thus absorbed may be esti-
mated at 10,000 grains (see Lesson VI. § 2).

134 ELEMENTARY PHYSIOLOGY. [less.

How much is taken in by the skin of man is not cer-
tainly known, but in some of the lower animals, such as
the frog, the skin plays a very important part in the
performance of the respiratory function.

26. The lyviphatic system has been already mentioned
as a feeder of the blood with a fluid which, in general,
appears to be merely the superfluous drainage, as it were,
of the blood-vessels : though at intervals, as we shall see,
the lacteals make substantial additions of new matter. It
is very probable that the multitudinous lyinphatic glaiids
may effect some change in the fluid which traverses them,
or may add to the number of corpuscles in the lymph.

Nothing certain is known of the functions of certain
bodies which are sometimes called ductless glands, but
have quite a different structure from ordinary secreting
glands ; and indeed do not resemble each other in struc-
ture. These are, the thyroid body, which lies in the part
of the throat below the larynx, and is that organ which,
when enlarged by disease, gives rise to " Derbyshire
neck " or " goitre ^' ; the iliymus body, situated at the
base of the heart, largest in infants, and gradually disap-
pearing in adult, or old persons ; and the supra-renal
bodies, which lie above the kidneys.

27. We are as much in the dark respecting the office of
the large viscus called the spleen, which lies upon the left
side of the stomach in the abdominal cavity (Fig. 38). It
is an elongated, flattened, red body, abundantly supplied
with blood by an artery called the splenic artery, which
proceeds almost directly from the aorta. The blood which
has traversed the spleen is collected by the splenic vein,
and is. carried by it to the 7^e?ia porter, and so to the liver.

A section of the spleen shows a dark red spongy mass
dotted over with minute whitish spots. Each of these
last is the section of one of the spheroidal bodies called
corpuscles of the spleen, which are scattered through its
substance, and consist of a solid aggregation of minute
bodies, like the white corpuscles of the blood, traversed
by a capillary network, which is fed by a small twig of
the splenic artery. The dark red part of the spleen, in
which these white spots are embedded, is composed of
a spongy framework of fibrous and elastic tissue, fre-
quently mixed with plain muscular fibres, and of peculiar

v.]

THE SPLEEN.

135

delicate vascular structures, which fill up the meshes of
the framework, and through which the splenic blood flows.

The elasticity of the splenic tissue allows the organ to
be readily distended with blood, and enables it to return
to its former size after distension. It appears to change
its dimensions with the state of the abdominal viscera,
attaining its largest size about six hours after a full meal,
and falling to its minimum bulk six or seven hours later,
if no further supply of food be taken.

The blood of the splenic vein is found to contain pro-
portionally fewer red corpuscles, but more colourless
corpuscles, than in the splenic artery ; and it has been

vcr \

Ao.

I>7n

The spleen {Spt) with the splenic artery {Sp A.). Below this is seen the
splenic vein running to help to form the vena portse {V.P.). Ao, the aorta.
D, a pillar of the diaphragm ; P.D, the pancreatic duct exposed by dissec-
tio'n in the substance of the pancreas : Dtn, the duodenum ; B.D, the
biliary- duct uniting with the pancreatic duct into the common duct, x ; y,
the intestinal vessels.

supposed that the spleen is one of those parts of the
economy in which, on the one hand, colourless corpuscles
of the blood are produced, and, on the other, red corpuscles
die and are broken up.

28. It has been seen that Jieat is being constantly given
off from the integument and from the air-passages : and

136 ELEMENTARY PHYSIOLOGY. [less.

everything that passes from the body carries away with it,
in like manner, a certain quantity of heat. Furthermore,
the surface of the body is much more exposed to cold than
its interior. Nevertheless, the temperature of the body is
in health maintained very evenly, at all times and in all
parts, within the range of two degrees or even less on
either side of 99"^ Fahrenheit.

This is the result of three conditions : — the first, that
heat is constantly being generated in the body ; the
second, that it is as constantly being distributed through
the body ; the third, that it is subject to incessant
regulation.

Heat is generated whenever oxidation takes place. As
we have seen, the tissues all over the body, muscle, brain-
substance, gland cells and the like, are continually under-
going oxidation. The living substance of the tissue, built
up out of the complex proteids, fats, and carbo-hydrates,
and thus even still more complex than these, is, by means
of the oxygen brought by the arterial blood, oxidised, and
broken down into simpler more oxidised bodies, which
are eventually reduced to urea, carbonic acid, and water.
Wherever life is being manifested these oxidative changes
are going on, more energetically in some places, in some
tissues, and in some organs, than in others ; and similar
changes, though perhaps not to any very great extent, are
taking place in the blood itself. Hence every capillar)- vessel
and every extra-vascular islet of tissue is really a small
fireplace in which heat is being evolved, in proportion to
the activity of the chemical changes which are going on.

29. But as the vital activities of different parts of the
body, and of the whole body, at different times, are very
different ; and as some parts of the body are so situated
as to lose their heat by radiation and conduction much
more easily than others, the temperature of the body
would be very unequal in its different parts, and at different
times, were it not for the arrangement by which the heat
is distributed and regulated.

Whatever oxidation occurs in any part, raises the tem-
perature of the blood which is in that part at the time, to
a proportional extent. But this blood is swiftly hurried
away into other regions of the body, and rapidly gives up
its increased temperature to them. On the other hand.

v.] TEMPERATURE OF THE BODY. 137

the blood which, by being carried to the vessels in the skin
on the surface of the body begins to have its temperature
lowered by evaporation, radiation, and conduction, is
hurried away, before it has time to get thoroughly cooled,
into the deeper organs ; and in them it becomes warm by
contact, as well as by the oxidating processes there going
on. Thus the blood-vessels and their contents might be
compared to a system of hot-water pipes, through which
the warm water is kept constantly circulating by a pump ;
while it is heated not by a great central boiler as usual,
but by a multitude of minute gas jets, disposed beneath
the pipes, not evenly, but more here and fewer there. It
is obvious that, however much greater might be the heat
applied to one part of the system of pipes than to another,
the general temperature of the water would be even
throughout, if it were kept moving with sufficient quick-
ness by the pump.

30. If such a system were entirely composed of closed
pipes, the temperature of the water might be raised to any
extent by the gas jets. On the other hand, it might be
kept down to any required degree by causing a larger, or
smaller, portion of the pipes to be wetted with water, which
should be able to evaporate freely— as, for example, by
wrapping them in wet cloths. And the greater the quantity
of water thus evaporated, the lower would be the tem-
perature of the whole apparatus.

Now, the regulation of the temperature of the human
body is effected on this principle. The vessels are closed
pipes, but a great number of them are inclosed in the
skin and in the mucous membrane of the air-passages,
which are, in a physical sense, wet cloths freely exposed to
the air. It is the evaporation from these which exercises
a more important influence than any other condition upon
the regulation of the temperature of the blood, and, conse-
quently, of the body.

But, as a further nicety of adjustment, the wetness of
the regulator is itself determined, through the aid of the
nervous system, by the temperature of the body. The
sweat-glands are so constituted that they are stimulated
to activity by warmth and rendered inactive by cold.
When the body is exposed to a high temperature (and the
same occurs when a part only of the body is heated) the

138 ELEMENTARY PHYSIOLOGY. [less.

action of certain nerv^es causes the sweat-glands to pour
forth a copious secretion on to the skin ; and when the
temperature falls, the glands cease to act. Moreover, in
this work of secreting sweat, the sweat-glands are assisted
by corresponding changes in the blood-vessels of the skin.
It has been stated (see Lesson IL, § 23) that the small
arteries of the body may be sometimes narrowed or con-
stricted, and sometimes widened or dilated. Now the con-
dition of the small arteries, whether they are constricted or
dilated, depends, as we have also seen, upon the action of
certain nerves (vaso-motor nerves). And it appears that
when the body is exposed to a high temperature these nerves
are so affected as to lead to a dilatation of small arteries
of the skin ; but when these are dilated the capillaries
and small veins in which they end become much fuller of
blood, and from these filled and swollen capillaries much
more nutritive matter passes through the capillary walls
to the sweat-glands, so that these have more abundant
material from which to manufacture sweat. On the other
hand, when the body is lowered in temperature the vaso-
motor nerves are so affected that the small arteries of the
skin are constricted ; hence less blood enters the capil-
laries of the skin, and less material is brought to the
sweat-glands.

Thus when the temperature is raised two things happen,
both brought about by the nervous system. In the first
place, the arteries of the skin are widened so that a much
larger proportion of the total blood of the body is carried
to the surface of the skin and there becomes cooled ; and,
secondly, this cooling process is greatly helped by the
increased evaporation resulting from the increased action of
the sweat-glands, whose activity is further favoured by the
presence in the skin of so much blood. Conversely when
the temperature is lowered, less of the blood is brought to
the skin, and more of the blood circulates through the
deeper, hotter parts of the body, and the sweat-glands
cease their work (this quiescence of theirs being in turn
favoured by the lessened blood- supply) ; hence the
evaporation is largely diminished, and thus the blood is
much less cooled.

Hence it is that, so long as the surface of the body per-
spires freely, and the air-passages are abundantly moist, a

v.]

TEMPERATURE OF THE BODY.

139

Fig. 39.— a Diagram to illustrate the Structure of Glands.

^A. Typical structure of the mucous membrane, a, an upper, and b a lower,
layer of epithelium cells ; c, the dermis with e, a blood-vessel, and / con-
nective tissue corpuscles.

I40 ELEMENTARY rilVSIOLOGY. [less.

man may remain with impunity, for a considerable time,
in an oven in which meat is being cooked. The heat of
the air is expended in converting this superabundant per-
spiration into vapour, and the temperature of the man's
blood is hardly raised.

31. Among the sources of loss to the blood which come
into operation at intervals only, the most important are
the glands proper, all of which are, in principle, narrow
pouches of the mucous membranes, or of the integument
of the body, lined by a continuation of the epithelium, or
of the epidermis. In the glands of Lieberkiih)i, which
exist in immense numbers in the walls of the small intes-
tines, each gland is nothing more than a simple blind sac
of the mucous membrane, shaped like a small test-tube,
with its closed ends outwards, and its open end on the
inner surface of the intestine (Fig. 39, i). The sweat-
glands of the skin, as we have already seen, are equally
simple, blind, tube-like involutions of the integument, the
ends of which become coiled up. The sebaceous glands^
usually connected with the air sacs, are shorter, and their
blind ends are somewhat subdivided, so that the gland
is divided into a narrow neck and a more dilated and
sacculated end (Fig. 39, 5). The neck by which the gland
communicates with the free surface is called its duct.
More complicated glands are produced by the elongation
of the duct into a long tube, and the division and sub-
division of the blind ends into multitudes of similar tubes,
each of which ends in a dilatation (Fig. 39, 6). These
dilatations, attached to their branched ducts, somewhat
resemble a bunch of grapes. Glands of this kind are
called racetiiose. The salivary glands and the pancreas
are such glands.

Now, many of these glands, such as the salivary, and
the pancreas (with the perspiratory-, or sudoriparous glands,

B. The same, with only one layer of cells, a and h, the so-called basement
membrane between the epitheLiim a, and dermis c.

1. A simple tubular gland.

2. A tubular gland bifid at its bx-;e. In this and succeeding figures

the blood-vessels are omitted.

3. A simple saccular gland.

4. A divided saccular gland, with a duct, d.

5. A similar gland still more divided.

6. A racemose gland, part only being drawn.

v.] SECRETION BV GLANDS. 141

which it has been convenient to consider already;, are only
active when certain impressions on the nenous system
give rise to a peculiar condition of the gland, or of it^
vessels, or of both.

Thus the sight or smell, or even the thought of food,
will cause a flow of saliva into the mouth : the previously
quiescent gland suddenly pouring out its fluid secretion, as
a result of a change in the condition of the nervous system.
And, in animals, a salivarv* gland can be made to secrete
abundantly, by irritating a ner\-e which supplies the gland
and its vessels. This etTect may be shown by experimental
evidence to be the result of a direct influence of the nerve
on the cells of the gland. What takes place is somewhat
as follows. As we shall see (Lesson ^ H.^, whenever a
nerve is irritated, or '' stimulated,'' at any point, as for
example by an electric shock, a change takes place in the
condition of the substance of the nerve at the point of
irritation. This change is propagated from particle to
panicle of the nervous matter, and thus travels along the
nerve-iibres as a nervous inipulsc. When the nerve of the
salivar)' gland is irritated, the nenous impulse, thus
started, travelling along the nerve reaches the cells of the
gland and sets up, in turn, changes in their substance.
The chief result of these changes in the cells of the gland
is the fonnation of a certain quantity of salivary- fluid,
which, as the secretion of the gland, passes from the cells
into the ducts.

We shall see (Lesson VI L) that if a nene which goes to
a muscle is irritated, a nervous impulse is transmitted in
the same way to the substance of the fibres of the muscle,
and gives rise to chemical changes in that substance. One
result of these changes is the evolution of carbonic acid
(§ 3-)) ^vhich might, therefore, be called a secretion of the
muscle. In the case of the muscle the chemical changes are
accompanied by a change of form, the fibres shortening
and becoming correspondingly thicker, while the products
of the chemical changes are returned to the blood and are
spoken of as waste. In the secreting cell there is no
appreciable change of fonn, and the products of the
chemical change, which are conspicuous and important,
pass, not into the blood, but, accompanied by much water,
into the duct of the gland.

142 ELEMENTARY PHYSIOLOGY. [less.

In the salivary gland, as in the sudoriparous gland, this
direct action of the nerve upon the gland is further assisted
by the fact that the stimulation of the nerve leads at the
same time to a w^idening of the arteries of the gland,
whereby the active cells are supplied more richly with
material for manufacturing their secretion.

The liquids poured out by these glands are always very
poor in solid constituents, and consist largely of water.
Those poured on to the surface of the body are lost, but
those which are received by the alimentary canal are
doubtless in a great measure re-absorbed.

32. A great intermittent source of gain to the blood is
to be found in the muscles, every contraction of which is
accompanied by a pouring of certain waste products into
the blood. Even when they are apparently at rest the
muscles are always pouring waste matters into the blood ;
but the amount of material which they thus give back to
the blood is under the circumstances not greater than,
indeed, at times, perhaps less than, the amount of nutritive
material which they take from the blood ; the activity
of a muscle, however, greatly increases the proportion
of its waste products. That much of this waste is
carbonic acid is certain from the facts {a) that the blood
which leaves a contracting muscle is always highly venous,
far more so than that which leaves a c^uiescent muscle ;
{b) that muscular exertion at once immensely increases
the quantity of carbonic acid expired ; but whether the
amount of nitrogenous waste is increased under the
circumstances, or not, is a point yet under discussion.

VI.] ALIMENTATION. 143

LESSON VL

THE FUNCTION OF ALIMENTATION.

1. The great source of gain to the blood, and, except
the kings, the only channel by which altogether new
material is introduced into that fluid, putting aside the
altogether exceptional case of absorption by the skin, is
the alimentary cafial, the totality of the operations of
which constitutes the function of alimentatio7i. It will
be useful to consider the general nature and results of the
performance of this function before studying its details.

2. A man daily takes into his mouth and thereby intro-
duces into his alimentary canal, a certain quantity of solid
and liquid food, in the shape of meat, bread, butter, water,
and the like. The amount of chemically dry, solid matter,
which must thus be taken into the body if a man of
average size and activity is neither to lose, nor to gain,
in weight, has been found to be about 8,000 grains. In
addition to this, his blood absorbs by the lungs about
10,000 grains of oxygen gas, making a grand total of
18,000 grains (or nearly two pounds and three-quarters
avoirdupois) of daily gain of dry, solid and gaseous
matter.

3. The weight of dry solid matter passed out from the
alimentary canal does not, on the average, amount to more
than one-tenth of that which is taken into it, or 800 grains.
Now the alimentary canal is the only channel by which
any appreciable amount of solid matter leaves the body in
an undissolved condition. It follows, therefore, that in

144 ELEMENTARY PHYSIOLOGY. [less.

addition to the 10,000 grains of oxygen, the equivalent
of 7,200 grains of dry, sohd, matter must pass out of the
body by the lungs, skin, or kidneys, either in the form of
gas, or dissolved in the liquid excretions of those organs.
Further, as the general composition of the body remains
constant, it follows either that the elementary constituents
of the solids taken into the body must be identical with
those of the body itself: or that, in the course of the
vital processes, the food alone is destroyed, the substance
of the body remaining unchanged : or, finally, that both
these alternatives hold good, and that food is, partly,
identical with the wasting substance of the body, and re-
places it ; and, partly, differs from the wasting substance,
and is consumed without replacing it.

4. As a matter of fact, all the substances which are
used as food come under one of four heads. They are
either what may be termed Profcids, or they are Fats,
or they are Amyloids, also called Carbohydrates, or they
are Minerals,

Proteids are composed of the four elements — carbon,
hydrogen, oxygen, and nitrogen, sometimes united with
sulphur and phosphorus.

Under this head come, the so-called Gluten of flour ; the
Albumin of white of egg, and blood serum ; the Fibrin of
the blood ; the substance, which is the chief constituent of
muscle and flesh, and which is called Myosin, or when
slightly altered, Syntoninj the Casein of milk and of
cheese, and many other similar but less common bodies ;
while Gelatin, which is obtained by boiling from connec-
tive tissue and by special means from bones, and Chon-
drifi, Avhich may be produced in the same way from
cartilage, may be considered to be outlying members of
the same group.

Fats are composed of carbon, hydrogen, and oxygen
only, and contain more hydrogen than is enough to form
water if united with the oxygen w'hich they possess.

All vegetable and animal fatty matters and oils come
under this division.

Amyloids or carbohydrates are substances which also
consist of carbon, hydrogen, and oxygen only. But they
contain no more hydrogen than is just sufficient to pro-
duce water with their oxygen. These are the matters

VI.] FOOD-STUFFS. ' 145

known as Starchy Dextrine^ Sugar; and closely allied
to them are the various Gujns.

It is the peculiarity of the three groups of food-stuffs
just mentioned that they can only be obtained (at any
rate, at present) by the activity of living beings, whether
animals or plants, so that they may be conveniently
termed vital foodstuffs.

Food-stuffs of the fourth class, on the other hand, or
Minerals, are to be procured as well from the not-living,
as the living world. They are water ^ and salts of sundry
alkalies, earths, and metals. To these, in strictness, oxy-
gen ought to be added, though, as it is not taken in by
the alimentary canal, it hardly comes within the ordinary
acceptation of the word food.

5. In ultimate analysis, then, it appears that vital food-
stuffs contain either three or four of the elements, carbon,
hydrogen, oxygen, and nitrogen ; and that mineral food-
stuffs are water and salts. But the human body, in ulti-
mate analysis, also proves to be composed of the same
four elements, plus water, and the same saline matters as
are found in food.

More than this, no substance can serve permanently
for food — that is to say, can prevent loss of weight and
change in the general composition of the body — unless it
contains a certain amount of proteid matter in the shape
of albumin, casein, &c., &c., while, on the other hand,
any substance which contains proteid matter in a readily
assimilable shape, is competent to act as a permanent
vital food-stuff.

The human body, as we have seen, contains a large
quantity of proteid matter in one or other of the forms
which have been enumerated ; and, therefore, it turns out
to be an indispensable condition, that every substance
which is to serve permanently as food, must contain a
sufficient quantity of the most important and complex
component of the body ready made. It must also con-
tain a sufficient quantity of the mineral ingredients which
are required. Whether it contains either fats or amyloids,
or both, its essential power of supporting the life and
maintaining the weight and composition of the body
remains unchanged.

6. The necessity of constantly renewing the supply of

1.6 ELEMENTARY PHYSIOLOGY. [less.

proteid matter arises from the circumstance that whether
the body is fed or not, a breaking down of proteid mate-
rial is continually going on, giving rise to a constant
nitrogenous waste, which leaves the body in the form of
urea. Now, this nitrogenous waste, coming from the
breaking down of proteid material, can only be met by
fresh proteid material being supplied. If proteid matter
be not supplied, the body must needs waste, because
there is nothing in the food competent to make good the
nitrogenous loss.

On the other hand, if proteid matter be supplied, there
can be no absolute necessity for any other but the mine-
ral food-stuffs, because proteid matter contains carbon
and hydrogen in abundance, and hence is competent to
make good not only the breaking down which is indicated
by the nitrogenous loss, but also t'lat which is indicated
by the other great products of waste, carbonic acid and
water.

In fact, the final results of the oxidation of proteid
matters are carbonic acid, water, and ammonia ; and
these, as we have seen, are the final shapes of the waste
products of the human economy.

7. From what has been said, it becomes readily intel-
ligible that, whether an animal be herbivorous or carni-
vorous, it begins to starve from the moment its vital
food-stuffs consist of pure amyloids, or fats, or any
mixture of them. It suffers from what may be called
7iitrogen starvation^ and, sooner or later, will die.

In this case, and still more in that of an animal deprived
of vital food altogether, the organism, so long as it con-
tinues to live, feeds upon itself In the former case, all
the processes involving a loss of nitrogen, in the latter,
all the processes leading to the appearance of all the
several waste products, are necessarily carried on at the
expense of its own body ; whence it has been rightly
enough observ^ed that a starving sheep is as much a car-
nivore as a lion.

8. But though proteid matter is the essential element
of food, and under certain circumstances may suffice by
itself to maintain the body, it is a very disadvantageous
i.'.nd uneconomical food.

Albumin, which maybe taken as a type of the proteids,

VI.] PROTEID DIET. 147

contains about 53 parts of carbon and 15 of nitrogen in
100 parts. If a man were to be fed on white of egg,
therefore, he would ta'ce in, speaking roughly, 3^^ parts of
carbon for every part of nitrogen.

But it is proved experimentally that a healthy, full-
grown man, keeping up his weight and heat, and taking
a fair amount of exercise, eliminates per diem 4,000 grains
of carbon to only 300 grains of nitrogen, or, roughly, only
needs one-thirteenth as much nitrogen as carbon. How-
ever, if he is to get his 4,000 grains of carbon out of
albumin, he must eat 7,547 grains of that substance.
But 7,547 grains of albumin contain 1,132 grains of
nitrogen, or nearly four times as much as he wants.

To put the case in another way, it takes about four
pounds of fatless meat (which generally contains about
one-fourth its weight of dry solid proteids) to yield 4,000
grains of carbon, whereas one pound will furnish 300
grains of nitrogen.

Thus a man confined to a purely proteid diet must eat
a prodigious quantity of it. This not only involves a
great amount of physiological labour in comminuting
the food, and a great expenditure of power and time in
dissolving and absorbing it, but throws a great quantity
of wholly profitless labour upon those excretory organs,
which have to get rid of the nitrogenous matter, three-
fourths of which, as we have seen, is superfluous.

Unproductive labour is as much to be avoided in phy-
siological as in political economy ; and it is quite possible
that an animal fed with perfectly nutritious proteid matter
should die of starvation ; the loss of power in various
operations required for its assimilation overbalancing the
gain ; or the time occupied in their performance being
too great to permit waste to be repaired with sufficient
rapidity. The body, under these circumstances, falls into
the condition of a merchant who has abundant assets,
but who cannot get in his debts in time to meet his
creditors.

9. These considerations lead us to the physiological
justification of the universal practice of mankind in adopt-
ing a mixed diet, in which proteids are mixed either with
fats or with amyloids, or with both.

Fats may be taken to contain about 80 per cent, of

L 2

148 ELEMENTARY PHYSIOLOGY. [less.

carbon, and amyloids about 40 per cent. Now it has
been seen that there is enough nitrogen to supply the
waste of that substance per diem, in a healthy man, in a
pound of fatless meat, which also contains 1,000 grains
of carbon, leaving a deficit of 3,000 grains of carbon.
Rather more than half a pound of fat, or a pound of
sugar, will supply this quantity of carbon,

10. Several apparently simple articles of food consti-
tute a mixed diet in themselves. Thus butcher's meat
commonly contains from 30 to 50 per cent, of fat. Bread,
on the other hand, contains the proteid gluten, and the
amyloids, starch and sugar, with minute quantities of fat.
But from the proportion in which these proteid and other
constituents exist in these substances, they are neither,
taken alone, such physiologically economical foods as
they are when combined in the proportion of about 200
to 75, or two pounds of bread to three-quarters of a pound
of meat per diem.^

11, It is quite certain that nine-tenths of the dry, solid
food which is taken into the body, sooner or later leaves
it in the shape of carbonic acid, water, and urea ; and it
is also certain not only that the compounds which leave
the body are more highly oxidised than those which enter
it, but that all the oxygen taken into the blood by the
lungs is carried away out of the body in the various waste
products.

The interm.ediate stages of this conversion are, how-
ever, by no means so clear. It is highly probable that
all the food-stuffs which pass from the alimentar\' canal
into the blood, be they proteids, or fats, or amyloids, be-
come part and parcel of some tissue or other (muscle,
nervous tissue, glandular tissue, and the like), before they
are oxidised ; that indeed it is as constituent elements of
some tissue or other that they suffer oxidation, and that

* It may be worth while to point out that mere chemical analysis is how-
ever, by itself, a ver>' insufficient guide as to the usefulness and nutritive
value of an ariicle of food. A substance to be nutritious must not only con-
tain some or other of the above food-stuffs, but contain them in an available,
that is a digestible form. A piece of beef-steak is far more nourishing, than
a quantity cf pease pudd.ng c mtaining even a larger proportion of proteid
material, because the former is far more digestible than the latter ; and a
small piece of dry hard cheese, though of high nutritive value_ as judged by
mere chemical analysis, will not satisfy the more subtle critidsm of the
stomach.

VI.] OXIDATION OF FOOD. 149

the amount of o.xidation going on in the blood is very
small. But this view, though probable, is not strictly
proved ; at all events, we cannot at present say exactly
how much oxidation takes place in the blood, or even
whether any takes place at all. Further, it is probable
that, under certain circumstances, the food may suffer
some amount of oxidation in the alimentar>' canal itself.

In the course of its oxidation, the food not only supplies
the energy which the body expends in doing work, but also
the energy which, as we have seen, the body loses as heat.
The oxidation of the food is indeed the ultimate source
of the heat of our bodies, all other causes being of little
moment. About this there can be no doubt, and it is fur-
ther probable that the oxidation which thus gives rise to
heat is not the oxidation of the elements of the food as
they are carried about in the blood, but the oxidation of
the tissues, more especially the muscles, into which the
food- stuffs have been built up, and of which they have
become an integral part.

12. Food-stuffs have been divided into heat-producers
and tissue-formers — the amyloids and fats constituting the
former division, the proteids the latter. But this is a very
misleading, and indeed erroneous classification, inasmuch
as it implies, on the one hand, that the oxidation of the
proteids does not develop heat ; and, on the other, that
the amyloids and fats, in being oxidised, subserve only
the production of heat.

Undoubtedly proteids are tissue-formers, inasmuch as no
tissue can be produced without them ; for all the tissues
are nitrogenous, some containing a large and others a
small quantity of nitrogen, and proteids are the only
nitrogenous food-stuffs ; they alone can supply the nitro-
genous elements of the tissues. But there is reason to
think that the fats and amyloids taken as food may also
be directly built up into the tissues. As we have seen,
when a muscle contracts, while there is abundant evidence
of carbonaceous waste, there is not such clear evidence
of nitrogenous waste ; that is to say, the non-nitrogenous
part of .the tissue seems to be used up more quickly than
the nitrogenous part ; and the consumption of this par-
ticular constituent of the muscular substance may be
made good by non-nitrogenous food, by fats or amyloids.

ISO ELEMENTARY PHYSIOLOGY. [less.

Cn the other hand, proteids must be regarded as heat-
producers also. Even if food be oxidised in the blood,
proteids, in being oxidised, will give rise to heat. And if
oxidation be, as has been suggested, largely confined to
the tissues, though in some tissues, as in muscles, the non-
nitrogenous part seems to be most rapidly changed, yet
the nitrogenous part, supplied by the proteids, is sooner
or later oxidised, and in being oxidised must give rise to
heat.

As soon as the elements of the food, in fact, get into
the blood, the distinction between the two classes is lost ;
both form tissues, and both supply heat.

If it is worth while to make a special classification of
the vital food-stuffs at all, it appears desirable to dis-
tinguish the essential food-stuffs, or proteids, from the
accessory food-stuffs, or fats and amyloids— the former
alone being, in the nature of things, necessar}' to life,
while the latter, however important, are not absolutely
necessary.

13. All food-stuffs being thus proteids, fats, amyloids,
or mineral matters, pure or mixed up with other sub-
stances, the whole purpose of the alimentary' apparatus is
to separate these proteids, &c., from the innutritious resi-
due, if there be any, and to reduce them into a condi-
tion either of solution or of excessively fine subdivision,
in order that they may make their way through the deli-
cate structures which form the walls of the vessels of the
alimentary canal. To these ends food is taken into the
mouth and masticated, is mixed with saliva, is swallowed,
undergoes gastric digestion, passes into the intestine, and
is subjected to the action of the secretions of the glands
attached to that viscus ; and, finally, after the more or less
complete extraction of the nutritive constituents, the resi-
due, mixed up with certain secretions of the intestines,
leaves the body as \}[i^ fcEces.

The cavity of the mouth is a chamber with a fixed roof,
formed by the \\?iX^ palate (Fig. 40, /}, and with a move-
able floor, constituted by the lower jaw, and the tongue {k'\
which fills up the space between the two branches of the
jaw. Arching round the margins of the upper and the
lower jaws are the thirty-two teeth, sixteen above and
sixteen below, and, external to these, the closure of the

VI.]

THE PALATE.

151

cavity of the mouth is completed by the cheeks at the
sides, and by the lips in front.

When the mouth is shut the back of the tongue comes
into close contact with the palate ; and, where the hard

Fig. 40.
A Section of the Mouth and Nose taken vertically, a little

TO THE LEFT OF THE MiDDLE LiNE.

a, the vertebral column ; i, the gullet ; c, the wind-pipe ; </, the thjToid
cartilage of the larynx ; £, the epiglottis ; /", the u\iila : ^, the opening of
the left Eustachian tube ; /*, the opening of the left lachr>-inal duct ; /, the
hyoid bone ; k, the tongue ; /, the hard palate ; tn, n, the base of the skull ;
o, p, q, the superior, middle, and inferior rurbinal bones. The letters ^,y, e,
are placed in the pharj-nx.

palate ends, the communication between the mouth and
the back of the throat is still further impeded by a sort of

152 ELEMENTARY I'llYSIOLOr.Y. [less.

fleshy curtain— the soft palate or velum — the middle of
which is produced into a prolongation, the uvula {/)■>
while its sides, skirting the sides of the passage, or fauces,
form double muscular pillars, which are termed ih.^ pillars
of the fauces. Between these the tonsils are situated, one
on each side.

The velum with its uvula comes into contact below with
the upper part of the back of the tongue, and with a sort
of gristly, lid-like process connected with its base, the
epiglottis {e).

Behind the partition thus formed lies the cavity of the
pharyfix, which may be described as a funnel-shaped bag
with muscular walls, the upper margins of the slanting,
wide end of which are attached to the base of the skull,
while the lateral margins are continuous with the sides,
and the lower with the floor, of the mouth. The narrow
end of the pharyngeal bag passes into the gullet or
oesophagus {d), a muscular tube, which affords a passage
into the stomach.

There are no fewer than six distinct openings into the
front part of the pharynx — four in pairs, and two single
ones in the middle line. The two pairs are, in front, the
hinder openings of the nasal cavities ; and at the sides,
close to these, the apertures of the Eustachian tubes (g).
The two single apertures are, the hinder opening of the
mouth between the soft palate and the epiglottis ; and,
behind the epiglottis, the upper aperture of the respira-
tory passage, or the glottis.

14. The mucous membrane which lines the mouth and
the pharynx is beset with minute glands, the buccal
glands J but the great glands from which the cavity of
the mouth receives its chief secretion are the three pairs
which, as has been already mentioned, are Z2^(t^ parotid^
submaxilla?y, sublingual, and which secrete the principal
part of the saliva (Fig, 41).

Each parotid gland is placed just in front of the ear,
and its duct passes forwards along the cheek, until it
opens in the interior of the mouth, opposite the second
upper grinding tooth.

The submaxillary and sublingual glands lie between the
lower jaw and the floor of the mouth, the submaxillary
being situated further back than the sublingual. Their

VI.]

SALIVA.

153

ducts open in the floor of the mouth below the tip of the
tongue. The secretion of these saHvary glands, mixed
with that of the small glands of the mouth, constitutes
the saliva— 1\ fluid which, though thin and watery, con-
tains a small quantity of animal matter, called Ftyalt?i,
which has certain very peculiar properties. It does not
act upon proteid food-stuffs, nor upon fats ; but if mixed
with starch, and kept at a moderate warm temperature, it
turns that starch into grape sugar. The importance of
this operation becomes apparent when one reflects that
starch is insoluble, and therefore, as such, useless as

Fig. 41-
A dissection of the right side of the face, showing, a, the sublingual, i5, the
submaxillary glands, with their ducts opening beside the tongue in the
floor of the mouth at d ; c, the parotid gland and its duct, which opens on
the side of the cheek at e.

nutriment, while sugar is highly soluble, and readily
passes through the walls of the alimentary canal.

15. Each of the thirty-two teeth which have been
mentioned consists of a crowji which projects above the
gum, and of one or more fangs, which are embedded in
sockets, or what are called alveoli, in the jaws.

The eight teeth on opposite sides of the same jaw are
constructed upon exactly similar patterns, while the eight

154 ELEMENTARY PHYSIOLOGY. [less.

teeth which are opposite to one another, and bite against
one another above and below, though similar in kind,
differ somewhat in the details of their patterns.

The two teeth in each eight which are nearest the
middle line in the front of the jaw, have wide but sharp
and chisel-like edges. Hence they are called incisors^
or cutting . teeth. The tooth which comes next is a
tooth with a more conical and pointed crown. It answers
to the great tearing and holding tooth of the dog, and is
called the ca}u?ie or eye-tooth. The next two teeth have
broader crowns, with two cusps, or points, on each crown,
one on the inside and one on the outside, whence they
are termed bicuspid teeth, and sometimes false grinders.
All these teeth have usually one fang each, except the
bicuspid, the fangs of which may be more or less com-
pletely divided into two. The remaining teeth have two
or three fangs each, and their crowns are much broader.
As they crush and grind the matters which pass between
them they are called molars, or true grinders. In the
upper jaw their crowns present four points at the four
comers, and a diagonal ridge connecting two of them.
In the lower jaw the complete pattern is five-pointed,
there being two cusps on the inner side and three on the
outer.

The muscles of the parts which have been described
have such a disposition that the lower jaw can be de-
pressed, so as to open the mouth and separate the teeth ;
or raised, in such a manner as to bring the teeth together ;
or more obliquely from side to side, so as to cause the
face of the grinding teeth and the edges of the cutting
teeth to slide over one another. And the muscles which
perform the elevating and sliding movements are of great
strength, and confer a corresponding force upon the
grinding and cutting actions of the teeth. In correspond-
ence with the pressure they have to resist, the superficial
substance of the crown of the teeth is of great hardness,
being formed of e?iamel, which is the hardest substance
in the body, so dense and hard, indeed, that it will strike
fire with steel (see Lesson XII.). But notwithstanding
its extreme hardness, it becomes worn down in old
persons, and, at an earlier age, in savages who live on
coarse food.

VI.] SWALLOWING. 155

16. When solid food is taken into the mouth, it is cut
and ground by the teeth, the fragments which ooze out
upon the outer side of their crowns being pushed beneath
them again by the muscular contractions of the cheeks
and lips ; while those which escape on the inner side are
thrust back by the tongue, until the whole is thoroughly
rubbed down.

While mastication is proceeding, the salivary glanas
pour out their secretion in great abundance, and the
saliva mixed with the food, which thus becomes inter-
penetrated not only with the salivary fluid, but with the
air which is entangled in the bubbles of the saliva.

When the food is sufficiently ground it is collected,
enveloped in saliva, into a mass or bolus, which rests
upon the back of the tongue, and is carried backwards to
the aperture which leads into the pharynx. Through this
it is thrust, the soft palate being lifted and its pillars being
brought together, while the backward movement of the
tongue at once propels the mass and causes the epiglottis
to incline backwards and downwards over the glottis,
and so to form a bridge by which the bolus can travel
over the opening of the air-passage without any risk of
tumbling into it. While the epiglottis directs the course
of the mass of food below, and prevents it from passing
into the trachea, the soft palate guides it above, keeps it
out of the nasal chamber, and directs it downwards and
backwards towards the lower part of the muscular pha-
ryngeal funnel. By this the bolus is immediately seized
and tightly held, and the muscular fibres contracting
above it, while they are comparatively lax below, it is
rapidly thrust into the oesophagus. By the muscular
walls of this tube it is grasped and propelled onwards, in
a similar fashion, until it reaches the stomach.

17. Drink is taken in exactly the same way. It does
not fall down the pharynx and gullet, but each gulp is
grasped and passed down. Hence it is that jugglers are
able to drink standing upon their heads, and that a horse,
or ox, drinks with its throat lower than its stomach, feats
which would be impossible if fluid simply fell down the
gullet into the gastric cavity.

During these processes of mastication, insalivation, and
deglutition, what happens to the food is, first, that it is

156 ELEMENTARY PHYSIOLOGY. [less.

reduced to a coarser or finer pulp ; secondly, that any
matters it carries in solution are still more diluted by
the water of the saliva ; thirdly, that any starch it may
contain begins to be changed into sugar by the peculiar
constituent (ptyalin) of the saliva.

1 8. The stomach, like the gullet, consists of a tube
with muscular walls composed of smooth muscular fibres,
and lined by an epithelium ; but it differs from the gullet
in several circumstances. In the first place, its cavity is
greatly larger, and its left end is produced into an enlarge-
ment which, because it is on the heart side of the body,
is called the cardiac dilatation (Fig. 42, b). The opening
of the gullet into the stomach, termed ^^ cardiac aperture,
is consequently nearly in the middle of the whole length of
the organ, which presents a long, convex, greater curva-
ture, along its front or under edge, and a short, concave,
lesser curvature, on its back or upper contour. Towards
its right extremity the stomach narrows, and, where
it passes into the intestine, the muscular fibres are so
disposed as to form a sort of sphincter around the
aperture of communication. This is called the Pylorus
(Fig. 42, d).

The mucous membrane lining the wall of the stomach
contains, or rather is made up of, a multitude of small
glands which open upon its surface. These are on the
whole simple in nature, being long tubular glands, but
they vary in character, their blind ends being more divided
and twisted at one part of the stomach than another.
Each gland is lined by an epithelium, the cells of (Fig.
43), which are of a peculiar nature and not all alike. It
is these, called gastric glands, which, when food passes
into the stomach, throw out a thin acid fluid, the gastric
juice.

When the stomach is empty, its mucous membrane is
pale and hardly more than moist. Its small arteries are
then in a state of constriction, and comparatively little
blood is sent through it. On the entrance of food a
nervous action is set up, which causes these small arteries
to dilate ; the mucous membrane consequently receives a
much larger quantity of blood, it becomes very red, little
drops of fluid gather at the mouth of the glands, and
finally run down as gastric juice. The process is very

VI.]

GASTRIC JUICE.

157

similar to the combined blushing and sweating which
takes place when the sympathetic in the neck is divided.

Pure gastric juice appears to consist of little more than
water, containing a few saline matters in solution, and its
acidity is due to the presence of free hydrochloric acid ;
it possesses, however, in addition a small quantity of a'
peculiar substance called /^^j/;/, which is a body in many
respects similar to, though very different in its effects
ixQTCi^ ptyalin (§ 14).

Fig. 42. — The Stomach Laid Open behind.
a, the oesophagus ; b, the cardiac dilatation ; c, the lesser curvature ; d, the
pylorus ; e, the biliary duct ; f, the gall-bladder ; g, the pancreatic duct,
cpening in common vith the cystic duct opposite h; h, i, the duodenum.

Thus, when the food passes into the stomach, the con-
tractions of that organ roll it about and mix it thoroughly
with the gastric juice.

• 19. It is easy to ascertain the properties of gastric juice
experimentally, by putting a small portion of the mucous
membrane of a stomach into acidulated water containing
small pieces of meat, hard-boiled egg, or other proteids,
and keeping the mixture at a temperature of about 100°.

ELEMENTARY PHYSIOLOGY.

[less.

After a few hours it will be found that the white of egg,
if not in too great quantity, has become dissolved : while
all that remains of the meat is a pulp, consisting chiefly
of the connective tissue and fatty matters which it con-
tained. This is artificial digesiioti, and it has been proved
by experiment that precisely the same operation takes place

Fig. 43.

One of the glands ^which secrete the gastric juice, magnified about
350 diameters.

when food undergoes natural digestion within the stomach
of a living animal.

Thus gastric juice dissolves proteids, and the proteid
solution thus effected is called a peptone^ and has pretty

VI.] PEPTONE. 159

much the same characters, whatever the nature of the
proteid which has been digested.

Peptone differs from all other proteids in its extreme
solubility, and in the readiness with which it passes
through animal membranes. Many proteids, as fibrin,
are naturally insoluble in water, and others, such as white
of egg, though apparently soluble, are not completely so,
and can be rendered quite solid or coagulated by being
simply heated, as when an egg is boiled. A solution of pep-
tone however is perfectly fluid, does not become solid, and
is not at all coagulated by boiling. Again, if a quantity
of albumin, such as white of egg or serum of blood, be tied
up in a bladder, and the bladder immersed in water, very
little of the proteid will pass through the bladder into the
water, provided that there are no holes. If, however, pep-
tone be used instead of albumin, a very large quantity will
speedily pass through into the water, and a quantity of
water will pass from the outside into the bladder, causing
it to swell up. This process is called osmosis, and is
evidently of great importance in the economy ; and the
purpose of the conversion of the various proteids by
digestion into peptone seems to be, in part at least, to
enable this class of food-stuff to pass readily into the
blood through the thin partition formed by the walls of
the mucous membrane of the intestine and the coats of
the capillaries. ^ Similarly, starch, even when boiled, and
so partially dissolved, will not pass through membranes,
whereas sugar does so with the greatest ease. Hence
the reason of the conversion of starch, by digestion, into
sugar.

It takes a very long time (some days) for the dilute acid
alone to dissolve proteid matters, and hence the solvent
power of gastric juice must be chiefly attributed to the
pepsin.

As far as we know gastric juice has no direct action on
fats ; by breaking up, however, the proteid framework in
which animal and vegetable fats are imbedded, it sets
these free, and so helps their digestion by exposing them
to the action of other agents. It appears, too, that gastric
juice has no direct action on amyloids ; on the contrary,
the conversion of the starch into sugar begun in the
mouth appears to be wholly or partially arrested by the

i6o

ELEMENTARY PIIYSTOLOGV. [less.

Fig. 44.— The Viscera of a Rabbit as seen upon simply opening the
Cavities of the Thorax and Abdomen withoct any further
Dissection.

A, cavity of the thorax, pleural cavity on either side; £, diaphragm;

VI.] THE INTESTINES. r6i

acidity of the contents of the stomach, ptyaHn being
active only in an alkahne or neutral mixture.

20. By continual rolling about, with constant additions
of gastric juice, the food becomes reduced to the con-
sistence of pea-soup, and is called chyme. In this state
it is, in part, allowed to escape through the pylorus and
to enter the duodenum ; but a great deal of the fluid
(consisting of peptone together with any saccharine fluids
resulting from the partial conversion of starch, or other-
wise) is at once absorbed, making its way, by imbibition,
through the walls of the delicate and numerous vessels of
the stomach into the current of the blood, which is rush-
ing though the gastric veins to the vetia porta".

21. The intestines form one long tube, with mucous and
muscular coats, like the stomach ; and, like it, they are
enveloped in peritoneum. They are divided into two por-
tions— the small in testifies and the large intestines; the
latter, though shorter, having a much greater diameter than
the former. The name of duodeniun^ is given to that
part of the small intestine which immediately succeeds
the stomach, and is bent upon itself and fastened by the
peritoneum against the back wall of the abdomen, in the
loop shown in Fig. 42. It is in this loop that the head
of the pancreas lies (Fig. 38).

The rest of the small intestines is no wider than the
duodenum, so that the transition from the small intestine
to the large (Fig. 45, a) is quite sudden. The opening of
the small intestine into the large is provided with promi-
nent lips which project into the cavity of the latter, and
oppose the passage of matters from it into the small
intestine, while they readily allow of a passage the other
way. This is the ileo-ccEcal valve (Fig. 45, d).

The large intestine forms a blind dilatation beyond the
ileo-caecal valve, which is called the cceciim ; and from
this an elongated, blind process is given ofl", which, from

C, ventricles of the heart ; D, auricles ; E, pulmonary arter>" ; F, aorta ;
G, lungs collapsed, and occupying only back part of chest ; H, lateral
portions of pleural membranes ; /, cartilage at the end of sternum (ensiform
cartilage) ; K, portion of the wall of body left between thorax and abdomen :
a, cut ends of the ribs ; Z, the liver, in this case lying more to the left than
the right of the body ; M, the stomach, a large part of the greater curvature
biing shown ; N, duodenum *, O, small intestine ; /", the caecum, so largely
developed in this and other herbivorous animals ; Q, the large intestine.

l62

ELEMENTARY rHYSIOLOGY

[less.

its shape, is called the I'crnuform appe?idix c\{ the caecum
(Fig. 45, b).

Fig. 45.

The termination of the illeum, a, in the caecum, and the continuation of the
latter into the colon, c; d, the ileo-caecal valve; e, the aperture of the
appendix 7>ert>n/or7nis (Ji) into the ca;cum.

Fig. 46. — Semi-diagrammatic View of Two Villi of the Small
Intestines. (Magnified about 50 diameters.)

a, substance of the villus ; h, its epithelium, of which some cells are seen
detached at />' ; c d, the artery and vein, with their connecting capillary
network which envelopes and hides c, the lacteal radicle which occupies
the centre of the villus and opens into a network of lacteal vessels at
its base. v

VI.] THE INTESTINES. 163

The cn?cum lies in the lower part of the right side of the
abdominal cavity. The colon^ or first part of the large
intestine,. passes upwards from it as the ascaidifi^ colon;
then making a sudden turn at a right angle, it passes
across to the left side of the body, being called the
transverse colon in this part of its course ; and next
suddenly bending backwards along the left side of the
abdomen, it becomes the descending colon. This reaches
the middle line and becomes the rectnin, which is that
part of the large intestine which opens externally.

22. The mucous membrane of the whole intestine is
provided with numerous small and simple glands
(named after Lieberkiihn), which pour into it a secretion,
the intestinal Juice, the precise functions of which are
unknown, though possibly in some animals it may possess
the power of converting starch into sugar, and proteids
into peptone. At the commencement of the duodenum
are certain racemose glands, called the glands of Brunner,
whose function seems unimportant.

Structures peculiar to the small intestine are the
valvule conniventes, transverse folds of the mucous
membrane which increase the surface ; and the villi,
which are minute club-shaped processes of the mucous
membrane set side by side, like the pile of velvet, over
the whole inner surface of the small intestine. Each
villus is a tongue-shaped projection of the mucous mem-
brane and has a covering of epithelium ; it contains in its
interior the lacteal radicle, or commencement of a lacteal
vessel (Lesson II. § 6), between which and the epithelium
lies a capillary network with its afferent artery and efferent
vein.

The intestines receive their blood almost directly from
the aorta. Their veins carry the blood which has tra-
versed the intestinal capillaries to the vena portce.

The fibres of the muscular coat of the intestines (which
lies between the mucous membrane and the serous, or
peritoneal, investment) are disposed longitudinally and
circularly ; the longitudinal coat being much thinner than,
and placed outside the circular coat. N ow the circular fibres
of any part contract, successively, in such a manner that
the upper fibres, or those nearer the stomach, contract
before the lower ones, or those nearer the large intestine

M 2

i64 ELEMENTARY PHYSIOLOGY. [less.

It follows from this so-called perisfaiiic contraction, that
the contents of the intestines are constantly being pro-
pelled, by successive and progressive narrowing of their
calibre, from their upper towards their lower parts. And
the same peristaltic movement goes on in the large
intestine from the ileo-csecal valve to the anus.

The large intestine presents noteworthy peculiarities
in the arrangement of the longitudinal muscular fibres of
the colon into three bands, which are shorter than the
walls of the intestine itself, so that the latter is thrown
into puckers and pouches ; and in the disposition of
muscular fibres around the termination of the rectum into
a ring-like sphincter muscle, which keeps the aperture
firmly closed, except when defecation takes place.

23. The only secretions, besides those of the proper
intestinal glands, which enter the intestine, are those of
the liver and the pancreas — the bile and the pa7icreatic
juice. The ducts of these organs have a common
opening in the middle of the bend of the duodenum ;
and, since the common duct passes obliquely through the
coats of the intestine, its walls serve as a kind of valve,
obstructing the flow of the contents of the duodenum
into the duct, but readily permitting the passage of bile
and pancreatic juice into the duodenum (Figs. 36, 38,42).
Pancreatic juice is an alkaline fluid not unlike saliva in
many respects ; it differs, however, in containing a con-
siderable quantity of proteid material. Bile we have
already studied.

After gastric digestion has been going on some time,
and the semi-digested food begins to pass on into the
duodenum, the pancreas comes into activity, its blood-
vessels dilate, it becomes red and full of blood, its cells
secrete rapidly, and a copious flow of pancreatic juice
takes place along its duct into the intestine.

The secretion of bile by the liver is much more con-
tinuous than that of the pancreas, and is not so markedly
increased by the presence of food in the stomach.
There is, however, a store of bile laid up in the
gall-bladder : and as the acid chyme passes into the
duodenum, and flows over the common aperture of the
gall and pancreatic ducts, a quantity of bile from this
reservoir in the gall-bladder is ejected into the intestine.

VI.] FAT EMULSIFIED. 165

The bile and pancreatic juice together here mix with the
chyme and produce remarkable changes in it.

24. In the first place, the alkali of these juices neutralises
the acid of the chyme ; in the second place, both the
bile and the pancreatic juice appear to exercise an
influence over the fatty matters contained in the chyme,
which facilitates the subdivision of these fats into very
minute separate particles. The fat, as it passes from
the stomach, is ver>' imperfectly mixed with the other
constituents of the chyme : and the drops of fat or oil
(for all the fat of the food is melted by the heat of the
stomach'^ readily run together into larger masses. By the
combined action, however, of the bile and pancreatic
juice the large drops of fat which pass into the intestine
from the stomach are emulsified^ that is to say are
broken up into exceedingly minute particles, and
thoroughly mixed with the rest of the contents : they
are brought in fact to ven.- much the same condition as
that in which fat {i.e. butter) exists in milk. When this
emulsifying has taken place the contents of the small
intestine no larger appear grey like the chyme in the
stomach but white and milky ; in fact it and milk are
white for the same reason, viz., on account of the multitude
of minute suspended fatty particles reflecting a great
amount of light.

The contents of the small intestine, thus white and
milky, are sometimes called chyle ; but it is best to reserve
this name for the contents of the lacteals, of which we
shall have to speak directly.

The emulsifying of the fats is not, however, the only
change going on in the small intestine. The pancreatic
juice has an action on starch similar to that of saliva, but
much more powerful. During the short stay in the mouth
ver>- little starch has had time to be converted into sugar,
and in the stomach, as we have seen, the action of the
saliva is arrested. In the small intestine, however, the
pancreatic juice takes up the work again ; and indeed, by
far the greater part of the starch which we eat is digested,
that is, changed into sugar, by the action of this juice.

Nor is this all, for, in addition to the above, the
alkaline pancreatic juice has a powerful effect on proteids
ver}- similar to that exerted by the acid gastric juice ;

i66 ELEMENTARY PHYSIOLOGY. [less.

it converts them into peptones, and the peptones so
produced do not differ materially from the peptones
resulting from gastric digestion.

Hence it appears that, while in the mouth amyloids only,
and in the stomach proteids only, are digested, in the
intestine all three kinds of food-stuffs, proteids, fats, and
amyloids, are either completely dissolved or minutely
.subdivided, and so prepared for their passage into the
vessels.

As the food is thrust along the small intestines by
the grasping action of the peristaltic conti^actions, the
digested matter which it contains is absorbed, that is,
passes away from the interior of the intestine into the
blood vessels and lacteals lying in the intestinal walls.

A great deal of this absorption takes place in the
small intestine (though the process is continued on in
the large intestine), and there can be no doubt that it is
largely effected by means of the villi. Each villus as we
have seen (§ 22), is covered by a layer of epithelium, and
contains in the centre a lacteal radicle, between which
and the epithelium lies a network of capillary blood-
vessels embedded in a delicate tissue. Now in some way
or other, not even yet thoroughly understood, the majority
of the minute particles of the finely divided, emulsified fat,
pass through the epithelium, past the capillary blood-
vessels, into the central lacteal radicle ; so that, after a
fatty meal, these lacteal radicles of the villi become filled
with fat. The lacteal radicle is continuous with the in-
terior of the lymphatic vessels which ramify in the walls
of the intestine, and which pass into the larger lymphatic
vessels running along the mesentery towards the thoracic
duct. Into these vessels the finely divided fat passes from
the lacteal radicle of the villus, and, mixing with the
ordinary lymph contained in these vessels, gives their
contents a white, milky appearance. Lymph thus white
and milky from the admixture of a large quantity of finely
divided fat is called chyle ; and this white chyle may after
a meal be traced along the lymphatics of the mesentery
to the thoracic duct, and along the whole course of that
vessel to its junction with the \enous system. After a
meal, in fact, this vessel is continually pouring into the
blood a large quantity of chyle, i.e. of lymph made white

VI.] ABSORPTION YKOyi THE INTESTINES. 167

and milky by the admixture of fats drawn from the vilH
of the small intestine.

The peptones and sugar, being soluble and diffusible,
pass, by a process which may be broadly described as
osmosis, through the epithelium into the substance of the
villi, and here they appear to be taken up by the capillary
blood-vessels of the villus, so that very little if any of
them gets to the lacteal radicle. From the capillaries
of the villi the peptones and sugar are then carried along
the z/c/ia portce to the liven where they probably undergo
some further change. So that while the fat, though it
gets for the most part into the general blood current by a
roundabout way, viz., by the lymphatics, reaches the blood,
as far as we know, very little changed, the peptones and
sugars on the other hand, though also taking a roundabout
course, viz., by the liver, are probably altered before they
are thrown into the general blood stream ; for the portal
blood in which they are carried is acted upon by the liver
before it flows through the hepatic vein into the general
venous system. But concerning both the process of
absorption itself and the changes undergone by the
absorbed products before they reach the heart, ready to
be distributed all over the body, we have probably much
yet to learn.

25. As the food thus passes along the small intestine,
digestion and absorption go on hand in hand. All the
way down, the proteids, amyloids, and fats of a meal are
being dissolved or freely divided, or otherwise changed,
and passing away into the lacteals or blood-vessels. So
that, by the time the contents of the intestine have reached
the ileo-caecal valve, a great deal of the nutritious matter
has been removed. Still, even in the large intestine,
some nutritious matter has still to be acted upon ; and we
find that, in the caecum and commencement of the large
intestine, changes are taking place, apparently somewhat
of the nature of fermentation, whereby the contents
become acid. In herbivorous animals it is probable that
ver}- considerable changes are effected in this part of the
alimentary canal.

One marked feature of the changes undergone in the
large intestine is the rapid absorption of water. Whereas
in the small intestine, the amount of fluid secreted into

i6S ELEMENTARY PHYSIOLOGY. [less.

the canal about equals that which is removed by absorp-
tion, so that the contents at the ileo-ca^cal valve are
about as fluid as they are in the duodenum ; in the large
intestine on the contrary, especially in its later portions,
the contents become less and fluid. At the same time a
characteristic odour and colour are developed, and the
remains of the food, now consisting either of undigestible
material, or of material which has escaped the action of
the several digestive juices, or withstood their influence,
gradually assume the characters of fseces.

yii.] THE HUMAN EDDY. 169

LESSON VII.

MOTION AND LOCOMOTION.

I. In the preceding Lessons the manner in which
the incomings of the human body are converted into its
outgoings has been explained. It has been seen that new
matter, in the form of vital and mineral food, is constantly-
appropriated by the body, to make up for the loss of
old matter, which is as constantly going on in the shape,
chiefly, of carbonic acid, urea, and water.

The vital foods are derived directly, or indirectly, from
the vegetable world : and the products of waste either
are such compounds as abound in the mineral world, or
immediately decompose into them. Consequently, the
human body is the centre of a stream of matter which
sets incessantly from the vegetable and mineral worlds
into the mineral world again. It may be compared to
an eddy in a river, which may retain its shape for an
indefinite length of time, though no one particle of the
water of the stream remains in it for more than a brief
period.

But there is this peculiarity about the human eddy,
that a large portion of the particles of matter which flow
into it have a much more complex composition than the
particles which flow out of it. To speak in what is not
altogether a metaphor, the atoms enter the body for the
most part, piled up into large heaps, and tumble down
into small heaps before they leave it. The energy which

lyo ELEMENTARY PHYSIOLOGY. [less.

they set free in thus tumbhng down, is the source of the
active powers of the organism.

2. These active powers are chiefly manifested in the
form of motion — movement, that is, either of part of the
body, or of the body as a whole, which last is termed
locomotion.

The organs which produce total or partial movements
of the human body are of three kinds : cells exhibiting
aincvboid niovcinentSj cilia, and uinscles.

The anurboid movements of the white corpuscles of the
blood have been already described, and it is probable that
similar movements are performed by many other simple
cells of the body in various regions.

The amount of movement to which each cell is thus
capable of giving rise may appear perfectly insignificant ;
nevertheless, there are reasons for thinking that these
amoeboid movements are of great importance to the
economy, and may under certain circumstances be followed
by very notable consequences.

3. Cilia are filaments of extremely small size, attached
by their bases to, and indeed growing out from, the free
surfaces of certain epithelial cells (see Lesson XI L) ; there
being in most instances very many (thirty for instance),
but, in some cases, only a few cilia on each cell. In some
of the lower animals, cells may be found possessing only
a single cilium. They are in incessant waving motion, so
long as life persists in them. Their most common form of
movement is that each cilium is suddenly bent upon itself,
becomes sickle-shaped instead of straight, and then more
slowly straightens again, both movements, however, being
extremely rapid and repeated about ten times or more
every second. These two movements are of course
antagonistic ; the bending drives the water or fluid in which
the cilium is placed in one direction, while the straightening
drives it back again. Inasmuch, however, as the bending
is much more rapid than the straightening, the force ex-
pended on the water in the former movement is greater
than in the latter. The total effect of the double move-
ment therefore is to drive the fluid in the direction towards
which the cilium is bent ; that is, of course, if the cell on
which the cilia are placed is fixed. If the cell be floating
free, the effect is to drive or row the cell backwards ; for

VII.] CILIA. 171

the cilia may continue their movements even for some
time after the epithehal cell, uith which they are connected,
is detached from the body. And not only do the move-
ments of the cilia thus go on independently of the rest of
the body, but they appear not to be controlled by the action
of the nen'ous system. Each cilium is comparable to one
of the mobile processes of a white corpuscle. A ciliated
cell differs from an amoeboid cell in that its contractile
processes are permanent, have a definite shape, and are
localised in a particular part of the cell, and that the
movements of the processes are performed rhythmically
and always in the same way. But the exact manner in
which the movement of a cilium is brought about is not
as yet thoroughly understood.

Although no other part of the body has any control
over the cilia, and though, so far as we know, they have
no direct communication with one another, yet their action
is directed towards a common end — the cilia, which cover
extensive surfaces, all working in such a manner as to
sweep whatever lies upon that surface in one and the same
direction. Thus, the cilia which are developed upon the
epithelial cells, which line the greater part of the nasal
cavities and the trachea, with its ramifications, tend to
drive the mucus in which they work, outwards.

In addition to the air-passages, cilia are found, in the
human body, in a few other localities ; but the part which
they play in man is insignificant in comparison with their
function in the lower animals, among many of which they
become the chief organs of locomotion.

4. Muscles (Lesson I. § 13) are accumulations of fibres,
each fibre having a definite structure which is difterent
in the striated and unstriated kinds (see Lesson XII.).
These fibres are bound up into small bundles by fibrous
(or connective) tissue, which carries the vessels and nerves ;
and these bundles are again similarly bound up together
in various ways so as to form muscles of various shapes
and sizes. Every fibre has the property, under certain
conditions, of shortening in length, while it increases
its other dimensions, so that the absolute volume of
the fibre remains unchanged. This property is called
tntisciilar tontracti/ity ; and whenever, in ^'irtue of this
property, a muscular fibre contracts^ it tends to bring

172 ELKMENTAKV PUVSlULOCiV. [less.

its two ends, with whatever may be fastened to them,
together.

The condition which ordinarily determines the con-
traction of a muscular fibre is, as we have seen (Lesson V.
§ 31), the passage along the nerve fibre, which is in close
anatomical connection with the muscular fibre, of a nervous
impulse^ i.e. of a particular change in the substance of the
nerve which is propagated from particle to particle along
the fibre. The nerve fibre is thence called a motor fibre,
because, by its influence on a muscle, it becomes the
indirect means of producing motion (Lesson XL § 6).

Muscle is a highly elastic substance. It contains a
large amount of water (about as much as the blood), and
during life has a clear and semi-transparent aspect.

When subjected to pressure in the perfectly fresh state,
and after due precautions have been taken to remove all
the contained blood, striated muscle (Lesson XI L § 15)
yields a fluid which undergoes spontaneous coagulation at
ordinary temperatures. At a longer or shorter time after
death this coagulation takes place wathin the muscles
themselves. They become more or less opaque, and,
losing their previous elasticity, set into hard, rigid masses,
which retain the form which they possess when the coagu-
lation commences. Hence the limbs become fixed in the
position in which death found them, and the body passes
into the condition of what is termed the " death- stiffening,"'
or rigor mortis. This stiffening is accompanied by a
change in the chemical reaction of the muscle, for while
living muscle, when tested with litmus is faintly alkaline
or neutral, at least when at rest, it becomes distinctly acid
as rigor mortis sets in. And it is a curious fact that a
similar acidity is developed even in a living muscle, when
it contracts.

After the lapse of a certain time the coagulated matter
liquefies, and the muscles pass into a loose and flaccid
condition, which marks the commencement of putre-
faction.

It has been observed that the sooner rigor mortis sets
in, the sooner it is over ; and the later it commences, the
longer it lasts. The greater the amount of muscular
exertion and consequent exhaustion before death, the
sooner rigor mortis sets in.

VII.] COMPOSITION OF MUSCLE. i73

Ri^or mortis evidently presents some analogies with the
coagulation of the blood, and the substance which thus
coagulates within the fibre {myosin (or muscle-clot) as it is
sometimes called) is in many respects not unlike fibrin.
It forms at least the greater part of the substance which
may be extracted from dead muscle by dilute acids, and
which is called synto}iin (see Lesson VI. § 4) Besides
myosin, muscle contains other varieties of proteid material
about which we at present know little ; a variable quantity
of fat ; certain inorganic saline matters, phosphates and
potash being, as is the case in the red blood-corpuscles,
in excess ; and a large number of substances existing in
small quantities, and often classed together as ' extrac-
tives.' Some of these extractives contain nitrogen ; the
most important of this class is k?'eati?i, a crystalline body
which is supposed to be the chief form in which nitro-
genous waste matter leaves the muscle on its way to
become urea.

The other class of extractives contains bodies free from
nitrogen, perhaps the most important of which are lactic
acid and glycogen.

Most muscles are of a deep, red colour ; this is due in
part to the blood remaining in their vessels ; but only in
part, for each fibre (into which no capillary enters) has
a reddish colour of its own, like a blood-corpuscle but
fainter. And this colour is probably due to the fibre
possessing a small quantity of that same haemoglobin in
which the blood-corpuscles are so rich.

5. Muscles may be conven'ently divided into two groups,
according to the manner in which the ends of their fibres
are fastened ; into muscles not attached to solid levers,
and muscles attached to solid levers.

Muscles not attached to solid levers. — Under this
head come the muscles which are appropriately called
hollow muscles, inasmuch as they inclose a cavity or sur-
round a space ; and their contraction lessens the capacity
of that cavity, or the extent of that space.

The muscular fibres of the heart, of the blood-vessels,
of the lymphatic vessels, of the alimentary canal, of the
urinary bladder, of the ducts of the glands, of the iris of
the eye, are so arranged as to form hollow muscles.

In the heart the muscular fibres which, though peculiar

t74 ELEMENTARY PHYSIOLOGY. [less.

are striated, are arranged in an exceedingly complex
manner round the several cavities, and they contract,
as we have seen, in a definite order.

The iris of the eye is like a curtain, in the middle of
which is a circular hole. The muscular fibres are of the
smooth or unstriated kind (see Lesson XIL), and they
are disposed in two sets : one set radiating from the edges
of the hole to the circumference of the curtain ; and the
other set arranged in circles, concentrically with the aper-
ture. The muscular fibres of each set contract suddenly
and together, the radiating fibres necessarily enlarging the
hole, the circular fibres diminishing it.

In the alimentary canal the muscular fibres are also of
the unstriated kind, and they are disposed in two layers ;
one set of fibres being arranged parallel with the length of
the intestines, while the others are disposed circularly, or
at right angles to the former.

As has been stated above (Lesson VI. § 22), the contrac-
tion of these muscular fibres is successive ; that is to say,
all the muscular fibres, in a given length of the intestines,
do not contract at once, but those at one end contract first,
and the others follow them until the whole series have
contracted. As the order of contraction is, naturally,
always the same, from the upper towards the lower end,
the etfect of this peristaltic contraction is, as we have seen,
to force any matter contained in the alimentary canal, from
its upper towards its lower extremity. The muscles of
the walls of the ducts of the glands have a substantially
similar arrangement. In these cases the contraction of
each fibre is less sudden and lasts longer than in the case
of the heart.

6. Muscles attached to definite level's. — The great ma-
jority of the muscles in the body are attached to distinct
levers, formed by the bones, the minute structure of which
is explained in Lesson XII. § 11. In such bones as
are ordinarily employed as levers, the osseous tissue is
arranged in the form of a sJiaft (Fig. 47, <5), formed of a
vtxy dense and compact osseous matter, but often contain-
ing a great central cavity {b) which is filled with a ver>'
delicate vascular and fibrous tissue loaded with fat called
niarroii'. Towards the two ends of the bone, the compact
matter of the shaft thins out, and is replaced by a much

VII.]

STRUCTURE OF A BONE.

175

thicker but looser sponge-work of bony plates and fibres,
which is termed the cancellous tissue of the bone. The

176 ELEMENTARY PHYSIOLOGY. [less.

surface even of this part, however, is still formed by a thin
sheet of denser bone.

At least one end of each of these bony levers is fashioned
into a smooth, articular surface, covered with cartilage,
which enables the relatively fixed end of the bone to play
upon the corresponding surface of some other bone with
which it is said to be articulated (see § 1 1), or, contrariwise,
allows that other bone to move upon it.

It is one or other of these extremities which plays the
part of fulcrum when the bone is in use as a lever.

Thus, in the accompanying figure (Fig. 48) of the bones
of the upper extremity, with the attachments of the biceps

Fig. 48.— The Bones of the Upper Extremity with the Biceps

Muscle.

The two tendons by which this muscle is attached to the scapula are seen
at a. P, indicates the attachment of the muscle to the radius, and hence
the point of action of the power ; F, the fulcrum, the lower end of the
humerus on which the upper end of the radius (together with the ulna)
moves ; W, the weight (of the hand).

muscle to the shoulder-blade and to one of the two bones
of the fore-arm called the radius, P indicates the point of
action of the power (the contracting muscle) upon the
radius.

But to understand the action of the bones, as levers,
properly, it is necessary to possess a knowledge of the
different kinds of levers and be able to refer the various

vir.]

LEVERS.

177

combinations of the bones to their appropriate lever-
classes.

A lever is a rigid bar, one part of which is absolutely or
relatively fixed, while the rest is free to move. Some one
point of the moveable part of the lever is set in motion
by a force, in order to communicate more or less of that
motion to another point of the moveable part, which pre-
sents a resistance to motion in the shape of a weight or
other obstacle.

Three kinds of levers are enumerated by mechanicians,
the definition of each kind depending upon the relative
positions of the point of support, ox fulcriuii ; of the point

Fig. 49.

IIL

The upper three figures represent the three kinds of levers *, the lower,

the foot, when it takes the character of each kind. — W, sveight or resist-
ance ; F, fulcrum ; P, power.

which bears the resistance^ iveight, or other obstacle to be
overcome by the force ; and of the point to which the
force, or poivcr employed to overcome the obstacle, is
applied.

If the fulcrum be placed between the power and the
weight, so that, when the power sets the lever in motion,
the weight and the power describe arcs, the concavities of
which are turned towards one another, the lever is said to
be of theyfrjr/ order. (Fig. 49, I.)

If the fulcrum be at one end, and tne weight be between
it and the power, so that weight and power describe con-
centric arcs, the weight moving through the less space

178 ELEMENTARY PHYSIOLOGY. [less.

when the lever moves, the lever is said to be of the second
order. (Fig. 49, II.)

And if, the fulcrum being still at one end, the power be
between the weight and it, so that, as in the former case,
the power and weight describe concentric arcs, but the
power moves through the less space, the lever is of the
third order. (Fig. 49, III.)

7. In the human body, the following parts present ex-
amples of levers of the first order.

{a) The skull in its movements upon the atlas, 2.% fulcrum.

\b) The pelvis in its movements upon the heads of the
thigh-bones, d.^ fulcrum.

ic) The foot, when it is raised, and the toe tapped on the
ground, the ankle-joint htmg fulcrum. (Fig. 49, I.)

The positions of the weight and of power are not given
in either of these cases, because they are reversed ac-
cording to circumstances. Thus, when the face is being
depressed, the power is applied in front, and the weight
to the back part, of the skull ; but when the face is being
raised, the power is behind and the weight in front. The
like is true of the pelvis, according as the body is bent
forward, or backward, upon the legs. Finally, when the
toes, in the action of tapping, strike the ground, the power
is at the heel, and the resistance in the front of the foot.
But when the toes are raised to repeat the act, the power
is in front, and the weight, or resistance, is at the heel,
being, in fact, the inertia and elasticity of the muscles and
other parts of the back of the leg.

But in all these cases, the lever remains one of the first
class, because the fulcrum, or fixed point on which the
lever turns, remains between the power and the weight, or
resistance.

8. The following are three examples of levers of the
second order : —

{a) The thigh-bone of the leg which is bent up towards
the body and not used, in the action of hopping.

For, in this case, the fulcrum is at the hip-joint. The
power (which may be assumed to be furnished by the thick
muscle ' of the front of the thigh) acts upon the knee-cap ;

* This muscle, called rectus, is attached above to the haunch-bone and
below to the knee-cap (Fisr 2, 2, p. 12). The latter bone is connected by a
strong ligament with the iibia.

VII.] EXAMPLES OF LEVERS. 179

and the position of the weight is represented by that of the
centre of gravity of the thigh and leg, which will lie some-
where between the end of the knee and the hip,

{b) A rib when depressed by the rectus muscle ' of the
abdomen, in expiration.

Here the fulcrum lies where the rib is articulated with
the spine ; the power is at the sternum— virtually the
opposite end of the rib ; and the resistance to be over-
come lies between the two.

{c) The raising of the body upon the toes, in standing
on tiptoe, and in the first stage of making a step forwards.
(Fig. 49, IL)

Here the fulcrum is the ground on which the toes rest ;
the power is applied by the muscles of the calf to the
heel (Fig. 2, L) ; the resistance is so much of the weight of
the body as is borne by the ankle-joint of the foot, which
of course lies between the heel and the toes.

9. Three examples of levers of the third order are —

{a) The spine, head, and pelvis, considered as a rigid
bar, which has to be kept erect upon the hip-joints.
(Fig. 2.)

Here the fulcrum lies in the hip-joints, the weight is high
above the fulcrum, at the centre of gravity of the head and
trunk ; the power is supplied by the extensor muscles (Fig.
2, 2) in the front of, or the flexor muscles (Fig. 2, H.) at
the back of, the thigh, and acts upon points comparatively
close to the fulcrum.

{b) Flexion of the forearm upon the arm by the biceps
muscle, when a weight is held in the hand.

In this case, the weight being in the hand and the ful-
crum at the elbow-joint, the power is applied at the point
of attachment of the tendon of the biceps, close to the
latter. (Fig. 48.)

{c) Extension of the leg on the thigh at the knee-joint.

Here the fulcrum is the knee-joint ; the weight is at the
centre of gravity of the leg and foot, somewhere between
the knee and the foot ; the power is applied by the muscles
in front of the thigh (Fig. 2, 2) through the ligament of the
knee-cap, ox patella, to the tibia, close to the knee-joint.

' This muscle lies in the front abdominal wall on each side of the middle
line. It is attached to the sternum above and to the front of the pelvis
below (I-'ig. 2, 3).

N 2

i8o ELEMENTARY THYSIOLOGY. [less.

10. In studying the mechanism of the body, it is very
important to recollect that one and the same part of the
body may represent each of the three kinds of levers,
according to circumstances. Thus it has been seen that
the foot may, under some circumstances, represent a lever
of the first, in others, of the second, order. But it may
become a lever of the third order, as when one dances a
weight resting upon the toes, up and down, by moving
only the foot. In this case, the fulcrum is at the ankle-
joint, the weight is at the toes, and the power is furnished
by the extensor muscles at the front of the leg (Fig. 2, i),
which are inserted between the fulcrum and the weight.

(Fig. 49, in.) , . , .'

11. It is very important that the levers of the body
should not slip, or work unevenly, when their movements
are extensive, and to this end they are connected together
in such a manner as to form strong and definitely-arranged
joints or articulations.

Joints may be classified into imperfect and perfect.

\a) Imperfect joints are those in which the conjoined
levers (bones or cartilages) present no smooth surfaces,
capable of rotatory motion, to one another, but are con-
nected by continuous cartilages, or ligaments, and have
only so much mobility as is permitted by the flexibility of
the joining substance.

Examples of such joints as these are to be met with in
the vertebral column — the flat surfaces of the bodies of
the vertebrae being connected together by thick plates of
very elastic fibro-cartilage, which confer upon the whole
column considerable play and springiness, and yet prevent
any great amount of motion between the several vertebrae.
In the pelvis (see Plate, Fig. VI.), the pubic bones are
united to each other in front, and the iliac bones to the
sacrum behind, by fibrous or cartilaginous tissue, which
allows of only a slight play, and so gives the pelvis a little
more elasticity than it would have if it were all one
bone.

{b) In all perfect joints y the opposed bony surfaces which
move upon one another are covered with cartilage, and
between them is placed a sort of sac, which lines these
cartilages, and, to a certain extent, forms the side walls
of the joint ; and which, secreting a small quantity of

vii.i PERFECT JOINTS. l8i

viscid, lubricating^ fluid — the synovia — is called :vsyno7'ial
incinbra)ic.

12. The opposed surfaces of these articular cartilages,
as they are called, may be spheroidal, cylindrical, or
pulley-shaped ; and the convexities of the one answer,
more or les^ completely, to the concavities of the other.

Sometimes, the two articular cartilages do not come
directly into contact, but are separated by independent
plates of cartilage, which are termed iniej'-ariicular. The
opposite faces of these inter-articular cartilages are fitted
to receive the faces of the proper articular cartilages.

While these co-adapted surfaces and synovial mem-
branes provide for the free mobility of the bones entering
into a joint, the nature and extent of their motion is de-
fined, partly by the forms of the articular surfaces, and
partly by the disposition of the ligaments, or firm, fibrous
cords which pass from one bone to the other.

13. As respects the nature of the articular surfaces,
joints may be what are called ball and socket joints, when
the spheroidal surface furnished by one bone plays in a
cup furnished by another. In this case the motion of the
former bone may take place in any direction, but the
extent of the motion depends upon the shape of the cup
— being very great when the cup is shallow, and small in
proportion as it is deep. The shoulder is an example of
a ball and socket joint with a shallow cup ; the hip, of
such a joint with a deep cup (Fig. 50),

14. Hinge-joints are single or double. In the former
case, the nearly cylindrical head of one bone fits into a
corresponding socket of the other. In this form of hinge-
joint the only motion possible is in the direction of a plane
perpendicular to the axis of the cylinder, just as a door
can only be made to move round an axis passing through
its hinges. The elbow is the best example of this joint in
the human body, but the movement here is limited, be-
cause the olecranon, or part of the ulna which rises up
behind the humerus, prevents the arm being carried back
behind the straight line ; the arm can thus be bent to, or
straightened, but not bent back (Fig. 51). The knee and
ankle present less perfect specimens of a single hinge-
joint.

A double hinge-joint is one in which the articular sur-

l82

ELEMENTARY PHYSIOLOGY.

[less.

face of each bone is concave in one direction, and convex
in another, at right angles to the former. A man seated
in a saddle is "articulated"' with the saddle by such a
joint. For the saddle is concave from before backwards,

Fig. 50. — A Section of the Hip-joint taken throi'gh the Aceta-
bulum OR Articular Cup of the Pelvis and the .middle of the

HEAD and neck OF THE ThIGH-BONE.

L. T. Ligamentum teres, or round ligament. The spaces marked with an
interrupted line (----) represent the articular cartilages. The cavity
of the s}*novial membrane is indicated by the dark line between these, and,
as is shown, extends along the neck of the femur beyond the limits of the
cartilage The peculiar shape of the pelvis causes the section to have the
remarkable outline shown in the cut. This w\\\ be intelligible if compared
with Fig. VI. in the plate.

and convex from side to side, while the man presents to it
the concavity of his legs astride, from side to side, and
the convexity of his seat, from before backwards.

The metacarpal bone of the thumb is articulated with

VII.]

riVOT JOINTS.

183

the bone of the wrist, called trapezium^ by a double hinge-
joint.

15. K pivot-joint is one in which one bone furnishes an
axis, or pivot, on which another turns ; or itself turns on
its own axis, resting on another bone. A remarkable
example of the former arrangement is afforded by the
atlas and axis^ or two uppermost vertebrae of the neck

Fig. 51.— LoNGiTL-niNAL and Vertical Section THRorou the
Elbow-joint.

H, humerus ; Ul, ulna , Tr, the triceps muscle, which e.xtends the arm ;
Bi, the biceps muscle, which flexes it.

(Fig. 52), The axis possesses a vertical peg, the so-called
odo7itoid process (^), and at the base of the peg are two,
obliquely placed, articular surfaces {a). The atlas is a
ring-like' bone, with a massive thickening on each side.
The inner side of the front of the ring plays round the
neck of the odontoid peg, and the under surfaces of the

1 84

ELEMENTARY PHYSIOLOGY.

[less.

lateral masses glide over the articular faces on each side
of the base of the peg. A strong ligament passes between
the inner sides of the two lateral masses of the atlas, and
keeps the hinder side of the neck of the odontoid peg
in its place (Fig. 52, A). By this arrangement, the atlas
is enabled to rotate through a considerable angle either
way upon the axis, without any danger of falling forwards
or backwards — accidents which would immediately destroy
life by crushing the spinal marrow.

The lateral masses of the atlas have, on their upper
faces, concavities (Fig. 52, A, a) into which the two con-
vex, occipital condyles of the skull fit, and in which they
play upward and downward. Thus the nodding of the

Fig.

52.

A. The atlas viewed from above ; a a, upper-articular surfaces of its lat-
eral masses for the condyles of the skull ; i>, the peg of the axis vertebra.

B. Side view of the axis vertebra ; a, articular surface for the lateral mass
of the atlas ; /', peg or odontoid process.

head is effected by the movement of the skull upon the
atlas ; while, in turning the head from side to side, the
skull does not move upon the atlas, but the atlas slides
round the odontoid peg of the axis vertebra.

The second kind of pivot-joint is seen in the forearm.

If the elbow and forearm, as far as the wrist, are made
to rest upon a table, and the elbow is kept firmly fixed,
the hand can nevertheless be freely rotated so that either
the palm, or the back, is turned directly upwards. When
the palm is turned upwards, the attitude is called supina-
tion (Fig. 53, A) ; when the back, pronation (Fig. 53, B).

The forearm is composed of two bones ; one, the u/na,
which articulates with the Jmmerus at the elbow by the

vir.]

PRONATION AND SUPINATION,

r85

hinge-joint already described, in such a manner that it can
move only in flexion and extension ^see § 17), and has no
power of rotation. Hence, when the elbow and wrist are
rested on a table, this bone remains unmoved.

But the other bone of the forearm, the radius, has its
small upper end shaped like a very shallow cup with thick

jr--

Fig. 53.

The bones of the right forearm in supination (A) and pronation (B).
//, humerus ; R, radius ; U, ulna.

edges. The hollow of the cup articulates with a sphe-
roidal surface furnished by the humerus : the lip of the
cup, with a concave depression on the side of the ulna.

The large lower end of the radius bears the hand, and
has, on the side next the ulna, a concave surface, which
articulates with the convex side of the small lower end of
that bone.

i86 ELEMENTARY PHYSIOLOGY. [less.

Thus the upper end of the radius turns on the double
surface, furnished to it by the pivot-hke ball of the hume-
rus, and the partial cup of the ulna ; while the lower end
of the radius can rotate round the surface furnished to it
by the lower end of the ulna.

In supination, the radius lies parallel with the ulna,
with its lower end to the outer side of the ulna (Fig. 53, A).
In pronation, it is made to turn on its own axis above, and
round the ulna below, until its lower half crosses the ulna,
and its lower end lies on the inner side of the ulna (Fig.
53, B).

16. The ligaments which keep the mobile surfaces of
bones together are, in the case of ball and socket joints,
strong fibrous capsules which surround the joint on all
sides. In hinge-joints, on the other hand, the ligamentous
tissue is chietly accumulated, in the form of lateral liga-
ments, at the sides of the joints. In some cases ligaments
are placed within the joints, as in the knee, where the
bundles of fibres which cross obliquely between the femur
and the tibia are called crucial ligaments ; or, as in the
hip, where the round ligament passes from the bottom of
the socket, or acetabulum of the pehis to the ball furnished
by the head of the femur (Fig. 50).

Again, two ligaments pass from the apex of the odon-
toid peg to both sides of the margin of the occipital
foramen, i.e. the large hole in the base of the skull, through
which the spinal cord passes to join the brain ; these, from
their function in helping to stop excessive rotation of the
skull, are called check ligaments (Fig. 54, a).

In one joint of the body, the hip, the socket or aceta-
bulum (Fig. 50) fits so closely to the head of the femur, and
the capsular ligament so completely closes its cavity on
all sides, that the pressure of the air must be reckoned
among the causes which prevent dislocation. This has
been proved experimentally by boring a hole through the
floor of the acetabulum, so as to admit air into its cavity,
when the thigh-bone at once falls as far as the round and
capsular ligaments will permit it to do, showing that it
was previously pushed close up by the pressure of the
external air.

17. The different kinds of movement which the levers
thus connected are capable of performing are called

VII.]

KINDS OF MOVEMENTS.

187

flexion and extension ; abduction and adductio)! j rotation
and ci7-cumduction.

A limb IS Jiexed, when it is bent ; extended, when it is
straightened out. It is abdncted, when it is drawn away
from the middle line ; addttcted, when it is brought to the
middle line. It is 7'otated, when it is made to turn on its
own axis ; circumducted, when it is made to describe a
conical surface by rotation round an imaginary axis.

No part of the body is capable of perfect rotation like
a wheel, for the simple reason that such motion would

The vertebral column in the upper part of the neck laid open to show, a,
the check ligaments of the axis ; b, the broad ligament which extends
from the front margin of the occipital foramen along the hinder faces of
the bodies of the vertebrse ; it is cut through, and the cut ends turned
back to show, r, the special ligament which connects the point of the
"odontoid" peg with the front margin of the occipital foramen; /, the
atlas ; //, the axis.

necessarily tear all the vessels, nerves, muscles, &c., which
unite it with other parts.

18. Any two bones united by a joint may be moved
one upon another in, at fewest, two different directions.
In the case of a pure hinge-joint, these directions must be
opposite and in the same plane ; but, in all other joints,
the movements may be in several directions and in
various planes.

i88 ELEMENTARY PHYSIOLOGY. [less.

In the case of a pure hinge-joint, the two practicable
movements — viz. flexion and extension — may be effected
by means of two muscles, one for each movement, and
running from one bone to the other, but on opposite sides
of the joint. When either of these muscles contracts, it
will pull its attached ends together, and bend or straighten,
as the case may be, the joint towards the side on which
it is placed. Thus the biceps muscle is attached, at
one end, to the shoulder-blade, while, at the other end,
its tendon passes in front of the elbow-joint to the radius
(Figs. 48 and 51,-5*/) : when this muscle contracts, therefore,
it bends, or flexes, the forearm on the arm. At the back of
the joint there is the triceps (7>-, Fig. 51) : when this con-
tracts, it straightens, or extends, the forearm on the arm.

In the other extreme form of articulation — the ball and
socket joint — movement in any number of planes may be
effected, by attaching muscles in corresponding number
and direction, on the one hand, to the bone which aftbrds
the socket, and on the other to that which furnishes the
head. Circumduction will be effected by the combined
and successive contraction of these muscles.

19. It usually happens that the bone to which one end
of a muscle is attached is absolutely or relatively sta-
tionar)', while that to which the other is fixed is movable.
In this case, the attachment to the stationary bone is
termed the origi?i^ that to the movable bone the insertiofi,
of the muscle.

The tibres of muscles are sometimes fixed directly into
the parts which serve as their origins and insertions ; but,
more commonly, strong cords or bands of fibrous tissue,
called tendons^ are interposed between the muscle proper
and its place of origin or insertion. When the tendons
play over hard surfaces, it is usual for them to be separated
from these surfaces by sacs containing fluid, which are
called burscE J or even to be invested by synovial sheaths,
i.e. quite covered for some distance by a synovial bag
forming a double sheath, very much in the same way that
the bag of the pleura covers the lung and the chest-wall.

Usually, the direction of the axis of a muscle is that of
a straight line joining its origin and its insertion. But in
some muscles, as the superior oblique muscle of the eye,
the tendon passes over a pulley formed by ligament, and

VII.]

WALKING.

189

completely changes its direction before reaching its inser-
tion. (See Lesson IX.)

Again, there are muscles which are fleshy at each end,
and have a tendon in the middle. Such muscles are called
digastric, or two-bellied. In the curious muscle which
pulls down the lower jaw, and especially receives this name
of digastric, the middle tendon runs through a pulley
connected with the hyoid bone : and the muscle, which
passes downwards and forwards from the skull to this
pulley, after traversing it, runs upwards and forwards to
the lower jaw (Fig. 55).

20. We may now pass from the consideration of the
mechanism of mere motion to that of locomotion.

Fig. 55. — The Col'rse of the Digastric Muscle.

D, its posterior belly ; D', its anterior belly ; bet^veen the two is the tendoQ
passing through its pulley connected with Hy, the hyoid bone.

When a man v. ho is standing erect on both feet pro-
ceeds to zjaik, beginning with the right leg, the body is
inclined, so as to throw the centre of gravity forward ;
and, the right foot being raised, the right leg is advanced
for the length of a step, and the foot is put down again.
In the meanwhile, the left heel is raised, but the toes of
the left foot have not left the ground when the right foot
has reached it, so that there is no moment at which both
feet are off the ground. For an instant, the legs form
two sides of an equilateral triangle, and the centre of the
body is consequently lower than it was when the legs
were parallel and close together.

I90 ELEMENTARY PHYSIOLOGY. [less.

The left foot, however, has not been merely dragged
away from its first position, but the muscles of the calf,
having come into play, act upon the foot as a lever of the
second order, and thrust the body, the weight of which
rests largely on the left astragalus, upwards, forwards, and
to the right side. The momentum thus communicated to
the body causes it, with the whole right leg, to describe an
arc over the right astragalus, on which that leg rests
below. The centre of the body consequently rises to its
former height as the right leg becomes vertical, and
descends again as the right leg, in its turn, inclines
forward.

When the left foot has left the ground, the body is
supported on the right leg, and is well in advance of the
left foot ; so that, without any further muscular exertion,
the left foot swings forward like a pendulum, and is carried
by its own momentum beyond the right foot, to the
position in which it completes the second step.

When the intervals of the steps are so timed that each
swinging leg comes forward into position for a new step
without any exertion on the part of the walker, walking
is effected with the greatest possible economy of force.
And, as the swinging leg is a true pendulum — the time of
vibration of which depends, other things being alike, upon
its length (short pendulums vibrating more quickly than
long ones), — it follows that, on the average, the natural
step of short-legged people is quicker than that of long-
legged ones.

In runfting, there is a period when both legs are off the
ground. The legs are advanced by muscular contraction,
and the lever action of each foot is swift and violent.
Indeed, the action of each leg resembles, in violent
running, that which, when both legs act together, consti-
tutes a jionp, the sudden extension of the legs adding
to the impetus, which, in slow walking, is given only by
the feet.

21. Perhaps the most singular motor apparatus in the
body is the larynx, by the agency of which voice is
produced.

The essential conditions of the production of the human
voice are : —

{a) The existence of the so-called vocdl chords.

VII.] THE LARYNX. 191

{b) The parallelism of the edges of these chords, without
which they will not vibrate in such a manner as to give
out sound,

{c) A certain degree of tightness of the vocal chords,
without which they will not vibrate quickly enough to
produce sound.

{d) The passage of a current of air between the parallel
edges of the vocal chords of sufficient power to set the
chords vibrating.

22. The lar}-nx is a short tubular box opening above
into the botton of the pharynx and below into the top of
the trachea. Its framework is supplied by certain carti-
lages more or less movable on each other, and these are
connected together by joints, membranes, and muscles.
Across the middle of the lan.-nx is a transverse partition,
formed by two folds of the lining mucous membrane,
stretching from either side, but not quite meeting in the
middle line. They thus leave, in the middle line, a chink
or slit, running from the front to the back, called the
glottis. The two edges of this slit are not round and
flabby, but sharp and, so to speak, clean cut ; they are
also strengthened by a quantity of elastic tissue, the fibres
of which are disposed lengthways in them. These sharp
free edges of the glottis are the so-called vocal chords or
vocal ligaments.

23. The thyroid Q2.ri\\2.<g^ Tig. 56, Th^ is a broad plate
of gristle bent upon itself into a V shape, and so disposed
that the point of the V is turned forwards, and constitutes
what is commonly called " Adam's apple."^ Above, the
thyroid cartilage is attached by ligament and membrane
to the hyoid bone (Fig. 56, Hy). Below and behind, its
broad sides are produced into little elongations or horns,
which are anicuiated by ligaments with the outside of a
great ring of cartilage, the cricoid Tig. 56, Cr>^ which
forms, as it were, the top of the windpipe.

The cricoid ring is much higher behind than in front,
and a gap, filled up by membrane only, is left between its
upper edge and the lower edge of the front part of the
thyroid, when the latter is horizontal. Consequently, the
thyroid cartilage, turning upon the articulations of its
horns with the hinder part of the cricoid, as upon hinges,
can be moved up and down through the space occupied by

192

ELEMENTARY PHYSIOLOGY

[less.

this membrane ; or, if the thyroid cartilage is fixed, the
cricoid cartilage moves in the same way upon its articula-
tions with the thyroid. When the thyroid moves' down-
wards or the cricoid upwards, the distance between the front
part of the thyroid cartilage and the back of the cricoid
is necessarily increased ; and when the reverse movement
takes place the distance is diminished. There is, on each
side, a large muscle, the crico-thyroid, which passes from
the outer side of the cricoid cartilage obliquely upwards

Fig. 56.

Diagram of the lar\-n.x, the thyroid cartilage {T)i) being supposed to be
transparent, and allowing the right arj-tenoid cartilage (^r), vocal chords
(F), and thyro-arj-tenoid muscle {ThA), the upper part of the cricoid
cartilage {Cr), and the attachment of the epiglottis {Ep) to be seen. CM,
the right crico-thyroid muscle ; Tr, the trachea ; Hy, the hyoid bone.

and backwards to the thyroid, and pulls the latter down ;
or, if the thyroid is fi.xed, pulls the cricoid up (Fig. 56,
C.th).

24. Perched side by side upon the upper edge of the
back part of the cricoid cartilage are two small irregularly-
shaped but, roughly speaking, pyramidal cartilages, the
arytejwid cartilages (Fig. 58, Ary^. Each of these is
articulated by its base with the cricoid cartilage by means
of a shallow joint which permits of very varied move-

VII.]

THE LARYNX.

193

ments, and especially allows the front portions of the two
ar^iienoid cartilages to approach, or to recede from, each
other.

It is to the forepart of one of these arytenoid cartilages
that the hinder end of each of the two vocal ligaments is
fastened ; and they stretch from these points horizontally
across the cavit)- of the lan,nx, to be attached, close to-
gether, in the re-entering angle of the thyroid cartilage
rather lower than half-way between its top and bottom.

Fig. 57. — Vertical and Transverse Section through the Larynx,

THE hinder half OF WHICH IS REMOVED.

Ep, Epiglottis ; Th, thj-roid cartilage ; a, cavities called the ventricles 0/
larynjc above the vocal ligaments (F); X the right thjTO-arjtenoid
muscle cut across ; Cr, the cricoid cartilage.

Now when the ar}-tenoid cartilages diverge, as they do
when the larynx is in a state of rest, it is evident that the
aperture of the glottis will be V-shaped, the point of the V
being forwards, and the base behind.

For, in front, or in the angle of the thyroid, the two
vocal ligaments are fastened permanently close together.

194

ELEMENTARY PHYSIOLOGY.

[less.

whereaSj behind, their extremities will be separated as far
as the arytenoids, to which they are attached, are separated
from each other. Under these circumstances a current
of air passing through the glottis produces no sound, the
parallelism of the vocal chords being wanting ; whence it
is that, ordinarily, expiration and inspiration take place
quietly. Passing from one ar}-tenoid cartilage to the
other, at their posterior surfaces are certain muscles
called \h^ posterior arytenoid (Fig. 58, Ar.p.). There are

s^-

^r./i.

Fig. 58. — The parts surrounding the Glottis partially dissected
and viewed from above.

Tk, the thyroid cartilage ; Cr, the cricoid cartilage ; /', the edges of the
vocal ligaments bounding the glottis ; A ry, the arytenoid cartilages ;
Th.A, thyro-ar^tenoid ; C.a.l, lateral crico-arj-tenoid ; C.a.p, posterior
crico-arytenoid ; A r.p, posterior arj'tenoid muscles.

also two sets of muscles connecting each arytenoid with
the cricoid, and called from their positions respectively
the posterior and lateral crico-arytenoid (Fig. 58, C.a.p.
C.a.l.). By the more or less separate or combined action
of these muscles, the arytenoid cartilages, and especially
the front part of these cartilages and, consequently, the
hinder ends of the vocal chords attached to them, may be
made to approach or recede from each other, and thus the
vocal chords rendered parallel or the reverse.

VII.]

THE LARYNX.

195

We have seen that the crico-thyroid muscle pulls the
thyroid cartilage down, or the cricoid cartilage up, and
thus increases the distance between the front of the
thyroid and the back of the cricoid, on which the
arytenoids are seated. This movement, the arytenoids
being fixed, must tend to pull out the vocal chords
lengthways, or in other words to tighten them.

Fig. 59.

I. View of the human larynx from above as actually seen by the aid of the
instrument called the laryngoscope ; A, in the condition when voice is
being produced ; B, at rest when no voice is produced.

e. Epiglottis (foreshortened).

c.v. The vocal chords.

c.v.s. The so-called false vocal chords, folds of mucous membrane ly'.ng

above the real vocal chords.
a. Elevation caused by the arytenoid cartilages.

5. lu. Elevations caused by small cartilages connected with the arytenoids.
/. Root of the tongue.

II. Diagram of the same.

Running from the re-entering angle in the front part
of the thyroid, backward, to the arytenoids, alongside the
vocal chords (and indeed imbedded in the transverse folds,
of which the chords are the free edges) are two strong
muscles, one on each side (Fig. 58, Th.A.), called fhyro-

o 2

196 ELEMENTARY PHYSIOLOGY. Ll£ss.

arytenoid. The effect of the contraction of these muscles
is to pull up the thyroid cartilage after it has been de-
pressed by the crico-thyroid muscles, (or to pull down
the cricoid after it has been raised,) and consequently to
slacken the vocal chords.

Thus the parallelism (b') of the vocal chords is deter-
mined chiefly by the relative distance from each other of
the arytenoid cartilages ; the tension (c) of the vocal cords
is determined chiefly by the upward or downward move-
ment of the thyroid or cricoid cartilage ; and both these
conditions are dependent on the action of certain muscles.

The current of air {d) whose passage sets the chords
vibrating is supplied by the movements of expiration,
which, when the chords are sufficiently parallel and tense,
produce that musical note which constitutes the voice, but
othenvise give rise to no audible sound at all.

25. Other things being alike, the musical note will be
low or high, according as the vocal chords are relaxed or
tightened : and this again depends upon the relative pre-
dominance of the contraction of the crico-thyroid and
thyro-ar\tenoid muscles. For when the thyro-arytenoid
muscles are fully contracted, the thyroid cartilage will be
raised, relatively to the cricoid, as far as it can go, and
the vocal chords will be rendered relatively lax ; while,
when the crico-thyroid muscles are fully contracted, the
thyroid cartilage will be depressed, relatively to the
cricoid, as much as possible, and the vocal chords will
be made more tense.

If, while a low note is being sounded, the tip of the
finger be placed on the crico-thyroid space (which can
be felt, through the skin, beneath the lower edge of the
thyroid cartilage;, and a high note be then suddenly pro-
duced, the crico-thyroid space will be found to be narrowed
by the approximation of the front edges of the cricoid and
thyroid cartilages. At the same time, however, the whole
lar)-nx is, to a slight extent, moved bodily upwards and
thrown forwards, and the cricoid has a particularly dis-
tinct upward movement ; this movement of the whole
larynx must be carefully distinguished from the motion
of the thyroid relatively to the cricoid.

The range of any voice depends upon the difterence of
tension which can be given to the vocal chords, in these

VII.]

RANGE vVND QUALITY OF VOICE.

197

two positions of the thyroid cartilage. Accuracy of
singing depends upon the precision with which the singer
can voluntarily adjust the contractions of the thyro-
arytenoid and crico-thyroid muscles — so as to give his
vocal chords the exact tension at which their vibration
will yield the notes required.

The quality of a voice — treble, bass, tenor, &c. — on the
other hand, depends upon the make of the particular
larynx, the primitive length of its vocal chords, their

Fig. 60.

Diagram of a model illustrating the action of the levers and muscles of the
larj-nx. The stand and vertical pillar represent the cricoid and arytenoid
cartilages, while the rod {pc), moving on a pivot at c, takes the place of the
thyroid cartilage ; a b is an elastic band representing the vocal ligament.
Parallel with this runs a cord fastened at one end to the rod b c, and, at the
other, passing over a pulley to the weight B. This represents the thyro-
arytenoid muscle. A cord attached to the middleof 3 c, and passing over a
second pulley to the weight A, represents the crico-thyroid muscle. It js
obvious that when the bar {b c)\s pulled down to the position c d, the elastic
band (« b) is put on the stretch.

elasticity, the amount of resonance of the surrounding
parts, and so on.

Thus, men have deeper notes than boys and Avomen,
because their larynxes are larger and their vocal chords
longer — whence, though equally elastic, they vibrate less
swiftly.

26. speech is voice modulated by the throat, tongue,
and lips, Thus, voice may exist without speech ; and it is

198 ELEMENTARY PHYSIOLOGY. [less.

commonly said that speech may exist without voice, as in
whispering. This is only true, however, if the title of voice
be restricted to the sound produced by the vibration of the
vocal chords ; for, in whispering, there is a sort of voice
produced by the vibration of the muscular walls of the
lips which thus replace the vocal chords. A whisper is,
in fact, a very low whistle.

The modulation of the voice into speech is effected by
changing the form of the cavity of the mouth and nose,
by the action of the muscles which move the walls of
those parts.

Thus, if the pure vowel sounds —

E (as in hc)^ A (as in hay), A' (as in aJi)^

O (as in or), 0' (as in <?//), 00 (as in coot),

are pronounced successively, it will be found that they
may be all formed out of the sound produced by a con-
tinuous expiration, the mouth being kept open, but the
form of its aperture, and the extent to which the hps are
thrust out or drawn in so as to lengthen or shorten the
distance of the orifice from the larynx, being changed for
each vowel. It will be narrowest, with the lips most
drawn back, in E, widest in A' , and roundest, with the
lips most protruded, m. 00.

Certain consonants also may be pronounced without
interrupting the current of expired air, by modification of
the form of the throat and mouth.

Thus the aspirate, H, is the result of a little extra ex-
piratory force — a sort of incipient cough. S and Z, Sh
and J (as \\\ jugular =G soft, as in gentry), Th, Z, A', F,
P^,may likewise all be produced by continuous currents of
air forced through the mouth, the shape of the cavity of
which is peculiarly modified by the tongue and lips.

27. All the vocal sounds hitherto noted so far resemble
one another, that their production does not involve the
stoppage of the current of air which traverses either of the
modulating passages.

But the sounds of M and N' can only be formed by
blocking the current of air which passes through the
mouth, while free passage is left through the nose. For

VII.] SPEECH. 199

J/, the mouth is shut by the hps ; for JV, by the appHcation
of the tongue to the palate.

28. The other consonantal sounds of the English
language are produced by shutting the passage through
both nose and mouth ; and, as it were, forcing the expira-
tor)' vocal current through the obstacle furnished by the
latter, the character of which obstacle gives each consonant
its peculiarity. Thus, in producing the consonants B
and P, the mouth is shut by the lips, which are then forced
open in this explosive manner. In T and D^ the mouth
passage is suddenly barred by the application of the point
of the tongue to the teeth, or to the front part of the palate ;
while in K and G (hard, as in go) the middle and back
of the tongue are similarly forced against the back part of
the palate.

29. An artificial lar\-nx may be constructed by properly
adjusting elastic bands, which take the place of the vocal
chords ; and, when a current of air is forced through these,
due regulation of the tension of the bands will give rise to
all the notes of the human voice. As each vowel and
consonantal sound is produced by the modification of the
length and form of the cavities, which lie over the natural
larynx, so, by placing over the artificial larynx chambers
to which any requisite shape can be given, the various
letters may be sounded. It is by attending to these facts
and principles that various speaking machines have been
constructed.

30. Although the tongue is credited with the respon-
sibility of speech, as the " unruly member,"' and undoubtedly
takes a ver)- important share in its production, it is not
absolutely indispensable. Hence, the apparently fabulous
stories of people who have been enabled to speak, after
their tongues had been cut out by the cruelty of a tyrant,
or persecutor, may be quite true.

Some years ago I had the opportunity of examining a
person, whom I will call Mr. R., whose tongue had been
removed as completely as a skilful surgeon could perform
the operation. When the mouth was widely opened, the
truncated face of the stump of the tongue, apparently
covered with new mucous membrane, was to be seen,
occupying a position as far back as the level of the an-
terior pillars, of the fauces. The dorsum of the tongue

200 ELEMENTARY PHYSIOLOGY. [less.

was visible with diftculty ; but I believe I could discern
some of the circumvallate papillae upon it. None of these
were visible upon the amputated part of the tongue, which
had been preserved in spirit ; and which, so far as I could
judge, was about 2^ inches long.

When his mouth was open, Mr. R. could advance his
tongue no further than the position in which I saw it ; but
he informed me that, when his mouth was shut, the stump
of the tongue could be brought much more forward

Mr. R.'s conversation was perfectly intelligible ; and
such words as tlmik^ the, cow, kill, were well and clearly
pronounced. But tin became _/f«,.' tack, fack or pack ;
toll, pool; dog, thog-; dine, vine; dew, thew j cat, cut/;
mad, madfj goose, gooth j big, pig, bich, pick, with a
guttural ch.

In fact, only the pronunciation of those letters the
formation of which requires the use of the tongue was
affected ; and, of these, only the two which involve the
employment of its tip were absolutely beyond Mr. R.'s
powen He converted all fs^ and d's into fs, p's, v's, or
th's. Th was fairly given in all cases ; s and sh, I and r,
with more or less of a lisp. Initial gs and k's were good ;
but final g's were all more or less guttural. In the former
case, the imperfect stoppage of the current of air by the
root of the tongue was of no moment, as the sound ran on
into that of the following vowel ; while, when the letter
was terminal^ the defect at once became apparent.

VIII.] THE NERVES. 201

LESSON VIII.

SENSATIONS AND SENSORY ORGANS.

1. The agent by which all the motor organs (except the
cilia) described in the preceding Lesson are set at work,
is muscular fibre. But, in the living body, muscular fibre
is, as a rule, made to contract by a change (Lesson V.
§ 31) which takes place in the motor or efferent nerve,
which is distributed to it. This change again is generally
effected by the activity of the ce7itral nervojis organ, with
which the motor nerve is connected. The central organ
is thrown into activity, directly or indirectly, by the
influence of changes which take place in nerves, called
sensory or afferent, which are connected, on the one
hand, with the central organ, and, on the other hand,
with some other part of the body. Finally, the alteration
of the afferent nerve is itself produced by changes in
the condition of the part of the body with which it is
connected ; which changes usually result from external
impressions.

2. Sometimes the central organ enters into a state of
activity without our being able to trace that activity to any
direct influence of changes in afferent nerves ; the activity
seems to take origin in the central organ, and the
movements to which it gives rise are called ' sponta-
neous,' or ' voluntary.' Putting these cases on one side,
it may be stated that a movement of the body, or of a
part of it, is to be regarded as the effect of an influence

202 ELEMENTARY PHYSIOLOGY. [less.

(technically termed a stimulus or irritation) applied
directly, or indirectly, to the ends of afferent nerves^ and
giving rise to a modification of the condition of the par-
ticles or niolecules which form the substance of the nerve
fibres, i.e. to a molecular change., which is propagated
from molecule to molecule along the fibres to the central
nervous orgaji with which these are connected. The mole-
cular activity of the afferent nerve sets up changes of a
like order in the fibres and cells of the central organ ;
from these the disturbance is transmitted along the motor
nerz'es, which pass from the central organ to certain
muscles. And, when the disturbance in the molecular
condition of the efferent nerves reaches the endings of
those nerves in muscular fibres, a similar disturbance is
communicated to the substance of the muscular fibres,
whereby, in addition to the production of certain other
phenomena to some of which reference has already been
made (Lesson V. § 31), the particles of the muscular sub-
stance are made to take up a new position, so that each
fibre shortens and becomes thicker.

3. Such a series of molecular changes as that just
described is called a refiex action : the disturbance in the
afferent nerves caused by the irritation being as it were
reflected back, along the efferent nerves, to the muscles.
But the name is not a good one, since it seems to imply
that the molecular changes in the afferent nerv^e, the cen-
tral organ, and the efferent nerve are all alike, and differ
only in direction ; whereas there is reason to think that
they differ in many ways.

A reflex action may take place without our knowing
anything about it, and hundreds of such actions are
continually going on in our bodies without our being
aware of them. But it very frequently happens that
we learn that something is going on, when a stimulus
affects our afferent nerves, by having what we call a
feelijig or sensatio?i. We class sensations along with
emotions.^ and volitions, and thoughts., under the common
head of states of cotisciousjiess. But what consciousness
is, we know not ; and how it is that anything so remark-
able as a state of consciousness comes about as the result
of irritating nervous tissue, is just as unaccountable as any
other ultimate fact of nature.

VIII.] THE MUSCULAR SENSE. 203

4. Sensations are of ver>' various degrees of definiteness.
Some arise within ourselves, we know not how or where,
and remain vague and undefinable. Such are the sensa-
tions of iincojufortableness. oi faint?iess^ oi fatigue, or of
restlessness. We cannot assign any particular place to
these sensations, which are ver>' probably the result of
affections of the afferent nerves in general brought about
by the state of the blood, or that of the tissues in which
they are distributed. And however real these sensations
may be, and however largely they enter into the sum of
our pleasures and pains, they tell us absolutely nothing of
the external world. They are not only diffuse, but they
are also subjective sensations.

5. What is termed the 7nuscular sense is less vaguely
localised than the preceding, though its place is still inca-
pable of being very accurately denned. This muscular
sensation is the feeling of resistance which arises when
any kind of obstacle is opposed to the movement of the
body, or of any part of it; and it is something quite
different from the feeling of contact or even of pressure.

Lay one hand tlat on its back upon a table, and rest a
disc of cardboard a couple of inches in diameter upon the
ends of the outstretched fingers : the only result will be a
sensation of contact — the pressure of so light a body being
inappreciable. But put a two-pound weight upon the card-
board, and the sensation of contact will pass into what
appears to be a very different feeling, viz., that of pressure.
Up to this moment the fingers and arm have rested upon
the table ; but now let the hand be raised from the table,
and another new feeling will make its appearance — that of
resistance to ejffort. This feeling comes into existence
with the exertion of the muscles which raise the arm ; and
it is the consciousness of that exertion which goes by the
name of ' the muscular sense.'

Any one who raises or carries a weight knows well
enough that he has this sensation ; but he may be greatly
puzzled to say where he has it. Nevertheless, the sense
itself is ver)- delicate, and enables us to form tolerably
accurate judgments of the relative intensity of resistances.
Persons who deal in articles sold by weight are constantly
enabled to form ver\- precise estimates of the weight of
such articles by balancing them in their hands ; and in

204 ELEMENTARY PHYSIOLOGY. [less.

this case, they depend in a great measure upon the mus-
cular sense.

6. In the case of other sensations, each feehng arises
out of changes taking place in a definite part of the body,
is produced by a stimulus applied to that part of the
body, and cannot be produced by stimuli applied to other
parts of the body. Thus the sensations of taste and S7nell
are confined to certain regions of the mucous membrane
of the mouth and nasal cavities ; those of sight and
Juaring to the particular parts of the body called the eye
and the ear ; and those of touchy though arising over a
much wider area than the others, are nevertheless re-
stricted to the skin and to some portions of the membranes
lining the internal cavities of the body. Any portion of
the body to which a sensation is thus restricted is called
a se?is€-orga?L

It may be here remarked that in the case of the sensa-
tion of touch, the simple feeling of contact is accompanied
by information, not only as to what sense-organ, but also
as to what part of that sense-organ, is being affected.
\Vhen we touch a hot or a rough body with the tip
of a finger, we are aware not only that we are dealing
with a hot or a rough body, but also that the hot or rough
body is in contact with the tip of the finger ; we ' refer,' as
is said, the sensation to that part of the tip of the finger
which is being acted upon by the body in question. With
the other sensations the case is different. When we smell a
bad smell, though we know that we smell by the nose, we
do not consider that the smell arises in the nose ; we con-
clude that there is some object outside ourselves which is
causing the bad smell. We refer the origin of the sensa-
tion to some external cause, and that even when the sen-
sation is after all due to changes taking place in the nose
itself independently of external objects, as in the un-
pleasant odours which accompany certain diseases of the
nose. Similarly all our sensations of sight and of hearing
are referred to external objects ; and even in the case of
taste, when a lump of sugar is taken into the mouth, we
are simply aware of a sensation of sweetness and do not
associate that sensation of sweetness with any particular
part of the mouth, though, by the sense of touch, which
the inside of the mouth also possesses, we can tell prett3'

VIII.] SENSE-ORGAXS. 205

exactly whereabouts in the mouth the melting lump is
lying.

7. In these sensations, thus arising in special sense-
organs, and hence often spoken of as ' special ' sensations,
each sensation or feeling results from the application of a
particular kind of stimulus to its appropriate sense-organ ;
and, in each case, the structure of the sense-organ is
arranged in such a manner as to render that organ
peculiarly sensitive to its appropriate stimulus.

Thus the sensations of sight are brought about by the
action of the vibrations of theluminiferous ether ; and the
eye, or sense-organ of sight, is constructed in such a way
that rays of light which falling on any other part of the
body produce no appreciable effect, give rise to vivid
sensations when they fall upon it.

Further we may, with more or less completeness, dis-
tinguish in each sense-organ two parts : an essential part,
through which the agent producing the sensation (be it light,
a series of sonorous vibrations, a sapid or odorous
chemical substance, a change in temperature, or a varia-
tion in pressure), produces changes in certain structures
which are peculiarly associated with the delicate termina-
tions of the nerve distributed to the sense-organ ; and an
accessory part, not absolutely necessary to the sense but of
great usefulness inasmuch as it assists in bringing the
agent to bear, in the most efficient way, upon the essejitial
part. In the case of the eye and ear this accessor}- part
is extremely complicated, and indeed seems to form the
greater part of the whole sense-organ ; in the case of the
other senses it is much more simple.

The essential part of each sense-organ is in turn com-
posed of minute organs, which upon examination appear
to be in reality modified epithelial cells ; and the dehcate
terminations of the nerve filaments distributed to the
sense-organ may, with more or less distinctness, be traced
to these modified cells, in which indeed they seem t6 end.
These minute organs, these modified epithelial cells, may
be spoken of as sefise-orga?7ules ; they serve as inter-
mediators in each case between the physical agent of the
sensation and the sensor)' nerve. The physical agent is
by itself unable to produce in the fibres of the sensory
ner\-e those changes which, reaching the brain as nervous

2o6 ELEMENTARY PHYSIOLOGY. [less.

impulses, give rise to the special sensations. Thus, as we
shall presently see, rays of light falling upon the optic nerve
cannot give rise to a sensation of sight. The physical agent
must act first on the sense-organules, and these in turn
act upon the filaments of the nerve. Thus light falling
upon the sense-organules, situated in that essential part of
the eye called the retina, sets up changes in them, these
changes set up corresponding changes in the delicate ner\-e
filaments which with the sense-organules go to make up the
retina, and the changes in the ner\-e filaments propagated
along the optic nerve to the brain give rise, in the latter,
to sensations of sight.

Hence in the essential part of each sense-organ we have
to distinguish between the sense-organules, i.e. the modi-
fied epithelium, and the terminal expansion of the sensory
ner\-e ; and further, in each sense-organ, there is added to
this essential part a more or less complicated accessory
part

Lastly, in all these special sensations, there are cer-
tain phenomena which arise out of the structure of the
sense-organ, and others which result from the operation
of the central apparatus of the nervous system upon the
materials supplied to it by the sense-organ.

8. The sense of Touch (including that of heat and
cold) is possessed, more or less acutely, by all parts of
the free surface of the body, and by the walls of the
mouth and nasal passages.

Whatever part possesses this sense consists of a mem-
brane (integumentar)- or mucous) composed of a deep
layer made up of fibrous tissue containing a capillar)'
network, and of a superficial layer consisting of epithelial
or epidermic cells, among which are no vessels.

Wherever the sense of touch is delicate, the deep layer
is not a mere flat expansion, but is raised up into multi-.
tudes of small, close-set, conical elevations (see Fig.' 32),
which" are called papHlce. In the skin, the coat. of epi-
thelial or epidermic cells does not follow the contour of
these papillae, but dips down between them and forms a
tolerably even coat over them. Thus, the points of the
papillae are much nearer the surface than the general
plane of the deep layer whence these papillae proceed.
Loops of vessels enter the papillae, and sensor}' nerve-fibres

VIII.] TOUCH. 207

are distributed to them. In some cases the nerve-fibre
ends in a papilla in a definite organ, in what is called a
tactile corpuscle (see Lesson XI 1.) or in a similar body
called an e?id-bulb. Each of these organs consists essen-
tially of an oval or rounded swelling formed by a modi-
fication and enlargement of the delicate connective tissue
ensheathing the nerve-fibre ; in the middle of the swelling
the nerve fibre itself ends abruptly in a peculiar manner.
These bodies are especially found in the papillae of those
localities which are endowed with a very delicate sense of
touch, as in the tips of the fingers, the point of the tongue,
&c. ; and the papillae which contain tactile corpuscles
generally contain few or no blood-vessels.

The great majority, however, of the nerve-fibres going
to the skm do not end in any such definite organs. They
divide in the dermis into exceeding delicate minute
filaments, the course and ultimate terminations of which
are traced with the greatest difficulty. Some of the finest
filaments, however, appear to pass into the epidermis and
to be there lost among or possibly connected with some
of the epidermic cells, especially those of the lower layers.

9. It is obvious, from what has been said, that no
direct contact takes place between a body which is
touched and the sensory nerve, — a thicker or thinner layer
of epithelium, or epidermis, being situated between the
two. In fact, if this layer is removed, as when a surface
of the skin has been blistered, contact with the raw
surface gives rise to a sense of pain, not to one of touch
properly so called. Thus, in touch, the essential part of
the sense-organ consists either of certain epithelial or epi-
dermic cells of the general integument or of certain
structures contained in the tactile corpuscles, end bulbs,
and other similar organs which need not be considered
here. These epithelial cells, very slightly modified
apparently in the general skin, but more so in the tactile
corpuscles and end bulbs, are the sense-organules ; they
serve as intermediators between the physical agent — pres-
sure— and the terminal filaments of the sensory nerves. The
accessory part of the sense-organ of touch is very slightly
developed, being chiefly supplied by the variable number
and form of the papillae and the variable thickness and
character of the layers of epidermic cells.

2o8 ELEMENTARY PHYSIOLOGY. [less.

10. Certain very curious phenomena appertain to the
sense of touch ; some of these are probably in part due
to these varying anatomical arrangements, to the var)-ing
thickness of the epidermis, and to the abundance or
scantiness of special end-organs. Not only is tactile
sensibility to a single impression much duller in some
parts than in others — a circumstance which might in many
cases be accounted for by the different thickness of the
epidermic layer — but the power of distinguishing double
simultaneous impressions is ver)- different. Thus, if the
ends of a pair of compasses (which should be blunted
with pointed pieces of cork) are separated by only one-
tenth or one-twelfth of an inch, they will be distinctly felt
as two, if applied to the tips of the fingers ; whereas,
if applied to the back of the hand in the same way,
only one impression will be felt ; and. on the arm,
they may be separated for a quarter of an inch, and still
only one impression will be perceived.

Accurate experiments have been made in different
parts of the body, and it has been found that two points
can be distinguished by the tongue, if only one-twenty-
fourth of an inch apart ; by the tips of the fingers if
one-twelfth of an inch distant ; while they may be one
inch distant on the cheek, and even three inches on the
back, and still give rise to only one sensation.

11. The feeling of warmth, or cold, is the result of
an excitation of sensor)- ner^-es distributed to the skin,
which are possibly distinct from those which give rise
to the sense of touch. And it would appear that the
heat must be transmitted through the epidermic or epithe-
lial layer, to give rise to this sensation ; for, just as touch-
ing a naked ner\'e, or the trunk of a nerve, gives rise
only to pain, so heating or cooling an exposed nene, or
the trunk of a nerve, gives rise not to a sensation of heat
or cold, but simply to pain. Thus, if the elbow be dipped
into a mixture of ice and salt, the cold first at^ects the
skin of the elbow, giving rise to a sensation of cold at the
elbow, but afterwards attacks the trunk of the ulnar nerve,
which at the elbow lies not ver>- far below the skin ; and
this latter effect is felt as a sensation, not of cold, but of
pain. The pain, moreover, thus caused is not felt in the
trunk of the nerve at the elbow, where the cold is acting,

VIII.] TASTE. 209

but in the parts where the fibres of the nerve end, more
particularly in the little and ring fingers.

Again, the sensation of heat, or cold, is relative rather
than absolute. Suppose three basins be prepared, one
filled with ice-cold water, one with water as hot as can
be borne, and the third with a mixture of the two. If
the hand be put into the hot-water basin, and then
transferred to the mixture, the latter will feel cold ; but
if the hand be kept a while in the ice-cold Avater, and
then transferred to the very same mixture, this will feel
warm.

Like the sense of touch, the sense of warmth varies in
delicacy in different parts of the body. The cheeks are
very sensitive, more so than the lips ; the palms of the
hands are more sensitive to heat than their backs. Hence
a washerwoman holds her flat-iron to her cheek to test
the temperature, and one who is cold spreads the palms
of his hands to the fire.

12. The organ of the sense of Taste is the mucous
membrane w^hich covers the tongue, especially its back
part, and the hinder part of the palate. Like that of
the skin, the deep, or vascular, layer of the mucous
membrane of the tongue is raised up into papillae ; but
these are large, separate, and have separate coats of
epithelium. Towards the tip of the tongue they are
for the most part elongated and pointed, and are called
Jiliform; over the rest of the surface of the tongue
these are mixed with other larger papillae, with broad
ends and narrow bases, c^Wed fiingzf or 711 j but towards its
root there are a number of large papillae, arranged in the
figure of a V with its point backwards, each of which
is like a fungiform papilla surrounded by a wall. These
are the circiu7ivallaie papillae (Fig. 61, C.p^. The larger
of these papillae have subordinate small ones upon their
surfaces. They are very vascular, and they receive
nervous filaments from tw^o sources, the one the nerve
called glossopharyngeal, the other the gustatory, w^hich is
a branch of the Jifth nerve (see Lesson XL § 18). The
latter chiefly supplies the front of the tongue, the former
its back and the adjacent part of the palate : and there
is reason to believe that different taste sensations are
supplied by the two nerves.

210

ELEMENTARY PHYSIOLOGY.

[less.

Certain of the epithelium cells covering the tongue and
palate are modified in a peculiar way ; these frequently
occur in groups, being arranged somewhat like leaves in a
bud, forming the so-called taste buds. These peculiar cells
are the sense-organ ules of taste, and, with the delicate

Fig. 6i. — The Mouth widely opened to siiow the Tongue and

Palate.

Uv. the u\Tala ; Tn. the tonsil between the anterior and posterior pillars of
the fauces ; C.p. circumvallate papillae ; F.p. fungiform papillae. The
minute filiform papilla; cover the interspaces between these. On the right
side the tongue is partially dissected to show the course of the filaments of
the glossopharyngeal ner\'e, VIII.

terminations of the glossopharyngeal and gustator)-- ner\'e
which may be traced to them, constitute the essential
parts of the organ of taste. The tongue itself, which
by its movements brings the sapid substances into

VIII.] SMELL. 211

immediate contact with these modified epithehum cells,
may be regarded as the accessory part.

The great majority of the sensations we call taste, how-
ever, are in reality complex sen.sations, into which smell,
and even touch, largely enter. When the sense of smell
is interfered with, as when the nose is held tightly pinched,
it is very difficult to distinguish the taste of various ob-
jects. An onion, for instance, the eyes being shut, may
then easily be confounded with an apple.

13. The organ of the sense of Smell is the delicate
mucous membrane which lines the upper part of the nasal
cavities. In this part the mucous membrane is distinguished
from the rest of the mucous membrane of these cavities —
firstly, by the character of its cells and by possessing no
cilia ; secondly, by receiving a large ner\-ous supply from
the olfactor)-, or first, pair of cerebral nerves, as well as
a certain number of filaments of the fifth pair, whereas
the rest of the mucojis membrane is supplied from the fifth
pair alone.

Each nostril leads into a spacious nasal chamber, sepa-
rated, in the middle line, from its fellow of the other side,
by a partition, or septum^ formed partly by cartilage and
partly by bone, and continuous with that partition which
separates the two nostrils one from the other. Below, each
nasal chamber is separated from the cavity of the mouth
by a floor, the l3ony palate (Figs. 62 and 63) ; and when
this bony palate comes to an end, the partition is continued
down to the root of the tongue by a fleshy (Curtain, the
soft palate, which has been already described. The soft
palate and the root of the tongue together, constitute, under
ordinan,' circumstances, a moveable partition between the
mouth and the pharynx ; and it will be observed that the
opening of the larynx, the glottis^ lies behind the partition ;
so that when the root of the tongue is applied close to the
soft palate no passage of air can take place between the
mouth and the pharynx. But in the upper part of the
phar}-nx above the partition are the two hinder openings
of the nasal cavities (which are called the posterior iiares)
separated by the termination of the septum ; and through
these wide openings the air passes, with great readiness,
from the nostrils along the lower part of each nasal chamber
to the glottis, or in the opposite direction. It is by means

P 2

2^12

ELEMENTARY PHYSIOLOGY.

[less.

Fk;. 62.— Vertical Longitudinal Sections of the Nasal Cavity.

The upper figure represents the outer wall of the left nasal cavity; the
lower figure the right side of the middle partition, or septum (S/.) of the
nose, which forms the inner wall of the right nasal cavity. /. the olfactory
nerve and its branches ; K, branches of the fifth nerve ; J'a. the paKite,
which separates the nasal cavity from that of the mouth : .S". T. the superior
turbinal bone ; M. T. the middle turbinal ; /. T. the inferior turbinal. The
letter / is placed in the cerebral cavity ; and the partition on which the
olfactory lobe rests, and through which the filaments of the olfactorj- nerves
pass, is the cribriform plate.

VIII.]

SMELL.

213

of the passages thus freely open to the air that we breathe,
as we ordinarily do, with the mouth shut.

Each nasal chamber rises, as a high vault, far above the
level of the arch of the posterior nares — in fact, about as
high as the depression of the root of the nose. The upper-
most and front part of its roof, between the eyes, is formed
by a delicate horizontal plate of bone, perforated like a
sieve by a great many small holes, and thence called the

:477.

.yi. PL

Fig. 63. — A Transverse and Vertical Section of the Osseous Walls
OF THE Nasal Cavity taken nearly through the letter / in
the foregoing Figure.

Cr. the cribriform plate •,S.T., M. T. the chambered superior and middle tur-
binal bones on which and on the septum {Sp.") the filaments of the olfactory
nerve are distributed ; /. T. the inferior turbinal bone ; PL the palate ; An.
the antrum or chamber which occupies the greater part of the maxillary
bone and opens into the nasal cavity.

cribriform plate (Fig. 63, Cr.). It is this plate (with the
membranous structures which line its two faces) alone
which, in this region, separates the cavity of the nose from
that which contains the brain. The olfactory lobes, which
are directly connected with, and form indeed a part of, the
brain, enlarge at their ends, and their broad extremities
rest upon the upper side of the cribriform plate, sending
through it immense numbers of delicate filaments, the

514 ELEMENTARY PHYSIOLOGY. [less.

olfactory' nerves, which are distributed as follows (Fig.
62) :—

On each wall of the septum the mucous membrane
forms a flat expansion, but on the side walls of each nasal
cavity it follows the elevations and depressions of the
inner surfaces of what are called the upper and middle
turbinal, or spongy bones. These bones are called spongy
because the interior of each is occupied by air cavities
separated from each other by verj- delicate partitions only,
and communicating with the nasal cavities. Hence the
bones, though massive-looking, are really exceedingly
light and delicate, and fully deserve the appellation of
spong}' (Fig. 63).

Over these upper and middle turbinal bones, and on
both sides of the septum opposite to them, the mucous
membrane is specially modified, and receives the name
of olfactory mucous membrane ; and it is to this olfactory
mucous membrane that the filaments of the olfactor}'
nerve passing through the cribriform plate are distributed.

There is a third light scroll-like bone distinct from these
two, and attached to the maxillary bone, which is called
the inferior turbinal, as it lies lower than the other two,
and imperfectly separates the air passages from the proper
olfactory chamber (Fig. 62). It is covered by the ordinary
ciliated mucous membrane of the nasal passage, and
receives no filaments from the olfactory ner^-e (Fig. 62).

In the non-olfactor)' part of the nasal mucous mem-
brane the epithelium cells are ordinary ciliated epithelium
cells (see Lesson XII.) ; but in the olfactory part the cells
not only lose their cilia, but become peculiarly modified.
Many of them become very slender and rod-shaped,
and the delicate temiinations of the olfactory nerve
filaments appear to end in these modified epithelial cells,
which indeed are the sense-organules of the organ of
smell. The olfactory mucous membrane, with the fila-
ments of the olfactory nerve ending in it, thus constitutes
the essential part of the organ.

14. The accessory part of the organ may be described
as follows : —

From the arrangements which have been described,
it is clear that, under ordinary circumstances, the gentle
inspiratory and expiratory currents will flow along the

VIII. 1 HEARING. 21^

comparatively wide, direct passages afforded by so much
of the nasal chamber as lies below the middle turbinal ;
and that they will hardly move the air enclosed in the
narrow interspace between the septum and the upper
and middle spongy bones, which is the proper olfactory
chamber.

If the air currents are laden with particles of odorous
matter, these can only reach the olfactory membrane by
diffusing themselves into this narrow interspace ; and, if
there be but few of these particles, they will run the risk
of not reaching the olfactory mucous membrane at all,
unless the air in contact with it be exchanged for some of
the odoriferous air. Hence it is that, when we wish to
perceive a faint odour more distinctly, we sniff, or snuff
up the air. Each sniff is a sudden inspiration, the effect
of which must reach the air in the olfactory chamber at
the same time as, or even before, it affects that at the nos-
trils ; and thus must tend to draw a little air out of that
chamber from behind. At the same time, or immediately
afterwards, the air sucked in at the nostrils entering with
a sudden vertical rush, part of it must tend to flow directly
into the olfactory chamber, and replace that thus drawn
out.

The loss of smell which takes place in the course of a
severe cold may, in part, be due to the swollen state of
the mucous membrane which covers the inferior turbinal
bones, impeding the passage of odoriferous air to the
olfactory chamber.

15. The Ear, or organ of the sense of Hearing, is ver}'
much more complex than either of the sensor}' organs yet
described ; and in it both the essential and the accessor)'
parts are much more highly developed.

The essential part, on each side of the head, consists,
substantially, of a very peculiarly-formed membranous
bag. This bag, when the ear first begins to be formed, is
a simple round sac, but it subsequently takes on a very
complicated form, and becomes divided into several parts,
which receive special names. It is lodged in a cavity of
correspondingly intricate shape, hollowed out of a solid
mass of bone ^called from its hardness petrosal)^ which
forms part of the temporal bone, and lies at the base
of the skull. The sac, however, does not completely fill

2i6 ELEMENTARY PHYSIOLOGY. [less.

the cavity, so that a space is left between the bony
walls and the contained sac. This space, which is
continuous all round the sac, being interrupted at certain
places only where the membranous sac is attached to the
bony walls, contains a fluid provided by the lymphatics of
the neighbourhood, and called perilyniph.

The membranous sac, the walls of which consist
chiefly of connective tissue, is lined by an epithelium,
and contains a fluid of its own called endolymph.
The perilymph, it will be understood, is quite distinct
from tb.e endolymph, the two fluids being separated by
the walls of the membranous sac.

Over a great part of the interior of the membranous
sac the epithelium is simple in character, but at certain
places to be presently described it assumes special
features, being greatly thickened, and bearing hair-
like processes, or being otherwise modified, so as to be
easily affected by even such slight movements as the
vibrations which produce sound. Where these patches
or tracts of modified or auditory epithelium^ as it is
called, exist, the membranous sac is more closely attached
to the bony walls ; and branches of the eighth, acoustic
or auditory, nerve passing through channels in the bony
walls, through the tissue attaching the membranous sac
to the bony walls, and through the wall of the mem-
branous sac itself, come into peculiar relation with, and
end in, or among, the cells of these patches of auditory
epithelium. It is only to the places where the epithelium
is thus modified that filaments of the auditory nerA-e are
distributed.

What takes place in hearing may briefly be stated as
follows. The vibrations set up by a sounding body are
conducted, by the accessory apparatus to be presently
described, to the perilymph, and from thence through
the walls of the membranous sac to the endolymph.
As the vibrations travelling along the endolymph reach
those particular places where the epithelium is mo-
dified, and where the filaments of the auditor)' nen-e
end, they in some way or other affect the epithelium
cells. Through the intermediation of these cells the
delicate endings of the auditory ner\-e are stimulated,
so that molecular changes are set up in the substance

VIII.] THE MEMBRANOUS LABYRINTH. 217

of the nerve, and transmitted along the nerve from
particle to particle, until they reach that part of the
brain the molecular disturbance of which gives rise to
sensations of sound.

Thus, until the auditory epithelium is reached, that which
takes place in the ear when we hear a sound is simply a
transmission of vibrations of the same order as those
which are produced by the sounding body ; but the pro-
cesses which intervene between the epithelium and the
brain are not of the same kind ; here there is no trans-
mission of such vibrations, but what takes place is a series
of changes of nerve substance of the same order as, though

Fig. 64. — The Membranous Labyrinth, twice the natural size.

Ut. the Utriculus, or part of the vestibular sac, into which the semicircular
canals open ; A, A, A, the ampullae ; P. A. anterior vertical semicircular
canal ', P. V. posterior vertical semicircular canal ; H. horizontal semi-
circular canal. The sacculus is not seen, as in the position in which the
labyrinth is drawn the sacculus lies behind the utriculus. The white
circles on the ampullae of the posterior, vertical, and horizontal canals indi-
cate the cut ends of the branches of the auditory nerve ending in those
ampullae ; the branches to the ampulla of the anterior vertical canal are
seen in the spaces embraced by the canal, as is also the branch to the
utriculus.

perhaps not exactly like, those which are set up by the
action of a stimulus on any other nerve (see Lesson V.
§ 31, VIL § 4).

16. The membranous bag, as I have said, is not simple
but complicated ; it consists of several parts. In the first
place there is a somewhat oval sac, called the tdricuhis
(Fig. 64, 67.) into which open three hoop-like, semicircular
canals. Of these two are placed vertically, one directed
anteriorly, the other posteriorly, and are hence called the
anterior {P. A.) and posterior {P. V.) vertical semicircular
canals. The third is placed horizontally and directed

2l8

ELEMENTARY PHYSIOLOGY.

[less*

outwards, hence it is called the exterior horizontal semi^
circular ca?ial (Fig. 64, //). It will be observed that the
three canals thus lie in the three directions of space ; this
has nothing to do with judging the directions of sound, but
may possibly have a relation to other functions of the
canals. Each of these three hoops is dilated at one of
its two ends, where it opens into the utriculus, into what
is called an ampulla (Fig. 64, A^ A, A), the other end

ES.a

A..^

PH.C

CocU

Fig. 65.

Diagram to illustrate the endings of the auditory nerve in the membranous
labyrinth and cochlea. N.B. The drazuing- is diagrainvmtic.

A.N. auditory nerve dividing into several branches, and ending : — at A.S.C.
in the ampulla of the anterior vertical semicircular canal : P.S.C. do. pos-
terior vertical : E.S.C. do. external horizontal : U. in the utriculus: S. in
the sacculus. Coch, the ending all along the canalis cochlearis. AA''.
canal uniting the interior of utriculus with that of sacculus. C. canal
joining the sacculus to the canalis cochlearis.

having no ampulla. Thus there is one ampulla to each
canal. Those ends of the two vertical canals Avhich are
not dilated into ampullje join together (Fig. 65), before
they open into the utriculus.

On each ampulla is a ridge or crest, called crista
acustica, placed crosswise, and projecting into the cavity of
the canal. Each crest is formed partly by an infolding and
thickening of the connective tissue wall of the ampulla,

VIII.] THE MEMBRANOUS LABYRINTH. 219

and partly by a thickening of the epithelium, which here
has the peculiar characters already referred to. A similar
but oval patch of thickened, modified, auditory epithelium,
with a thickening of the wall beneath it, is found in, the
utriculus itself ; this is called a viacida aciistica.

Attached to the utriculus is a similar smaller sac (forming
another division of the primitive membranous bag) called
the sacculiis hemispJiericus^ on the walls of which is a
similar rounded patch of modified epithelium, or macula.
The cavity of the sacculus is cut off from tbat of the utri-
culus, except for a curious roundabout connection by
means of a narrow canal (Fig. 65, av^.

The utriculus and sacculus are often called the vestibule j
and with the three semicircular canals receive the name of
the inonbranous labyrinth. It will be remembered that this
membranous labyrinth, filled with endolymph, lies in an
intricate cavity with bony walls called the osseous labyrinth,
and that between the walls of the bony and the mem-
branous labyrinth, which corresponds largely but not
wholly in form, is a space filled with perilymph.

Branches of the auditory nerve pass to this mem-
branous labyrinth and send fibres (Fig. 65) to the three
crests of the three ampullae, to the patch on the utriculus,
and to the patch on the sacculus. In each crest and each
patch the epithelium is thickened and modified, and al-
though the crests are slightly different in structure from
the patches, the general features are the same in all.
Whereas over the rest of the inside of the membranous
labyrinth the epithelium consists (Fig. 66, e) of a single
layer of low, rather flat cells, in the crests and patches
the cells lie several deep, and are of a peculiar form.
Some are conical or cylindrical, and some are spindle-
shaped, and either the one or the other, or, according to
some authors, both, bear stiff hair-like filaments (Fig. 66,
a.h. A.B. a.h.) projecting into the cavity of the labyrinth.
These filaments, often called auditory hairs, appear at first
sight to resemble cilia, but they are stiff, and unlike cilia
have no active movement of their own. They are longer
and more conspicuous in the crests of the ampullae than
in the ]jatches of the utriculus and sacculus. The fibres
of the auditory nen-e may be traced through the con-
nective tissue wall of the crest or patch into the epithelium,

220

ELEMENTARY PHYSIOLOGY

[less.

-n, \c.l;

A B

Fig. 66.— LoNGiTiDiN-AL Section of Ampulla, cutting the Crest
Crosswise (somewhat diagrammatic).

<r, one end of the ampulla forming the semicircular canal, u, the other end
opening into the utricle ; e, ordinary epithelium hning the greater part
of the ampulla ; cr. The crest with a.e. auditor^' epithelium ; a.h.
auditor^' hairs; c.t. connective tissue support to the auditorj- epithe-
lium ; «, fihres of the auditory ner\-e passing into the auditorj' epithe-

VIII.] OTOLITHS. 221

where they break up into a dehcate network among the
cells (Fig. 66, A.B. b.); but it is not as yet exactly de-
termined how the filaments of this network end, whether
they actually join the conical cells, or the spindle cells, or
merely lie in contact with them.

However this may be, it is ver)- clear that the vibrations,
or waves of sound, reaching the ear from some sounding
body, in passing along the endol\Tnph,set in movement these
hairs, ver\- much as waves of the \\-ind set in movement
stalks of standing com, and that the movements of the
hairs, by help of the cells to which the hairs belong, excite
the delicate filaments of the nersous network below, and
so set up disturbances or impulses which pass along the
auditor}- nerve to the brain.

In the utriculus and sacculus where, as has been said,
the hairs are not so conspicuous, the endolymph contains
a number of small calcareous particles called otoliths, and
these are supposed b)' many to be of use in increasing the
eft'ect of the waves in the endolymph. In bathing in a
tolerably smooth sea, on a rocky shore, the movement of
the little waves as they run backwards and forwards is
hardly felt by any one hing down ; but in bathing on a
sandy and gravelly beach the pelting of the showers of
little stones and sand, which are raised and let fall by each
wavelet, makes a ver)- definite impression on the nen'es of
the skin. And it may be that the movements of these
otoliths in a similar way produce a greater effect on the
epithelium than would the mere waves of the endoh-mph ;
but in some of the lower animals these minute particles
are replaced by one large stone which seems rather to act

Hum ; i, epithelium intermediate between the auditory epithelium and the
ordinary' epithelium of the rest of the amptdla.

A and B. Diagrams to illustrate the character of the cells of the auditory
epithelium, and the two ^-iews taken as to the relation of the auditors-
hairs to the cells. In both A and B, I is the auditory epithelium, II the
connective tissue on which it rests, and a, a fibre of the auditory ner\-e
passing through II, and di%-iding into fine branching filaments in I, at b.

In h, c.c. cylindrical cells bearing auditorj- hairs, n.k. ; each cell bears a
group of fine hairs which adhere together as a long narrow cone ; s^.c.
spindle-shaped cells, not bearing hairs.

In B, c.c. cylindrical cells not bearing hairs, sfi.c. spindle-shaped cells bearing
the auditor>- hair, d, and supposed to be connected \v"ith the nerve-filaments ;
y'othet supporting cells.

In both A and B, the fibre, a, of the auditory- ner\'e passes into the epithelium,
and ends in fine branches, i.

222 ELEMENTARY PHYSIOLOGY. [less.

as a damper ; so that the exact use of the otohths must be
left at present undecided.

17. An important part of the essential apparatus
yet remains to be described, and that is the cochlea.

Connected with the sacculus by a narrow canal is an
extension of the original membranous sac, in the form of a
long tube closed at the end (Fig. 65, Coch.). This cochlear
tube, like the parts of the membranous sac already de-
scribed, is lined with epithelium, contains endolymph, and
is lodged in a bony cavity filled with perilymph. So far it
resembles the labyrinth, but in many other respects it is
ver)^ different.

In the first place, in the labyrinth, the membranous
sac ver)' closely follows the contour of the bony walls,
so that in a section of a semicircular canal, for instance,
the membranous canal presents a circular contour lying
in the larger circular contour of the bony canal. But in the
cochlea, on the contrar}^, the contour of the cochlear tube
is, along its whole length, totally different from that of
the containing cavity ; for, in transverse section, while
the contour of the containing cavity is almost circular,
that of the cochlear tube itself is nearly triangular. The
cochlear tube in fact is, in shape, what is often called
triangular (as when we speak of a triangular file), but
should be called tn7ied?'al ; that is to say it has three sides
or faces (and three edges) ; one of the sides is however
not flat but convex, i.e. bulges somewhat outwards.

In the second place, in the labyrinth, the sac is for the
most part free from the bony walls, being attached only at
the places where the nerve fibres pass into it, and, more
loosely, at some few other points ; but in the cochlea, on
the contrary, the cochlear tube closely adheres to the bony
wall, along the whole length of the tube, in two regions,
namely, over the whole of that face of the trihedral tube
which has just been described as being convex, and at
the edge opposite. Take a round ruler, make a paper
case which just fits it, and close the case at one end.
Then pare down the ruler on two sides until it has two
flat faces meeting at an edge, and slide it into the case,
so that it does not quite reach the closed end. The ruler,
if it were hollow, would represent the cochlear tube ; and
it will be observed that it divides the cavity of the case

VIII.]

THE COCHLEA.

223

into two passages, Avhich are quite distinct from each
other, except at the end of the case to which the ruler does
not reach. In a similar way, the cochlear tube, contain-
ing endolpnph, divides the cavit}- containing perilymph,
in which it lies, into two passages, called scalcB^ which are
seen in section (Fig. 67) to be placed one above and the
other below the triangular cavity of the cochlear tube
itself, and which communicate with each other at the far
end of the cochlear tube, but not elsewhere.

In one point, however, the comparison with the ruler
and its case is not exact. The cochlear tube is not nearly
so ^^-ide as the containing cavity : and the sharp edge

Fig. 67. — A Section* THRorcH the Axis of the Cochlea, magnified

THREE DIAMETERS.

Sc.M. scala media ; Sc. V. scala vestibuli ; Sc. T. scala tympani ; L.S. lamina
spiralis ; Md. bony axis, or modiolus, round which the scalae are wound ;
C.N. cochlear nerAe.

opposite the convex adherent face would not be in direct
connexion with the bony walls, were it not for a bony ledge
which, projecting from the bony walls towards the thin
edge of the cochlear tube, is united to it by membrane and
thus forms a partition or septKJH, which separates the two
scalae in the region where the cochlear tube itself would
otherwise leave a communication between them.

In the third place, the cochlear tube is not straight or
even simply cun-ed, but is twisted up on itself, into a spiral
of two and a half turns. In these twists it is accom-
panied by the cavities above and below it, and also by the
septum spoken of above, which thus takes a spiral course,

224 ELEMENTARY PHYSIOLOGY. [less.

and is spoken of as thj lamina spiralis (Figs. 67, 68, l.s.).
The whole arrangement somewhat resembles the shell
of a snail ; hence the name. All along the spiral the edge
of the cochlear tube attached to the lamina spiralis is
directed inwards and the convex face outwards ; so that
when a section is made through the axis of the spiral a
succession of rounded spaces are cut through, each space
exhibiting, above and below, the somewhat half-moon-
shaped section of a scala, the two scalse being separated
on the outer side, by the cochlear tube, and, on the inner,
by the lamina spiralis (Fig. 67).

The triangular cavity which, as we have seen, contains
endolymph, and is continuous with the sacculus, is called
the ca?ialis cochlearis, or scala media (because it lies
between the two other cavities). The upper of the two
cavities containing perilymph, when traced down to the
bottom of the spiral, is found to be continuous with the
cavity containing perilymph which surrounds the vestibule
{i.e. the utriculus and sacculus) ; hence it is called the
scala vestibuli. The lower cavity, when similarly traced
to the bottom of the spiral, ends against the inner wall of
a part of the ear to be presently described, called the
ty?npanu>?i, by an opening, called the fenestra rotunda,
which is closed by a membrane. Hence this lower cavity
is called the scala tynipani. Thus the scala vestibuli
and scala tympani begin at different points, and are
separated along their whole course by the cochlear tube
and the lamina spiralis except at the ver)' tip of the spiral,
where these latter end ; here the two scalar are prolonged
beyond the cochlear tube and join together, forming a
common space, as seen at the top of Fig. 67.

The vibrations of sound are brought, as we shall see, to
the perilymph chamber of the vestibule, whence they spread
on the one hand over the semicircular canals, and on the
other into the scala vestibuli. Passing upwards, in the
spiral along the scala vestibuli, they enter at the summit the
scala tympani, along which they descend, and are eventually
lost at the fenestra rotunda in which that scala ends.

18. But besides this peculiar arrangement of the perilymph
chamber, there are other and still more important differ-
ences between the cochlea and the labyrinth.

The auditory nerve is, as we have seen, distributed to

VIII.]

THE COCHLEA.
h

225

Tig. 68. — Section of Coil ov Cochlea.
Sc V scala vestibuli ; Sc. T. scala tympani ; C.C. canalis cochlearis, or

scaia media; O-C. organ of Corti ; m.R. membrane of Reissner, ni.t.

membrana tectoria (a gelatinous membrane overlying the organ of Corti,

and supposed to act as a damper). A.N. fibres of the auditory nerve

rcnn-ng in Is., the lamina spiralis, and endmg in the organ of Corti;

a, connective tissue cushion to which the basilar membrane is attached

on the outside ; b, bony walls.

The figure has, for simplicit>-'s sake, been made somewhat diagrammatic.
The lamina spiralis has been drawn too short ; the proportions of the lamina
spiralis and the scalse are more exactly rendered in Fig. 67.

226 ELEMENTARY PHYSIOLOGY. [less.

certain parts only of the membranous labyrinth, namely, to
the crests of the ampullar and to the patches on the utriculus
and the sacculus ; but, in the case of the cochlea, fibres,
running in canals excavated in the bony core of the spiral,
and in the lamina spiralis (Fig. 68, A.N.) run to and end
in the canalis cochlearis along its whole length, from the
bottom to the top of the spiral. Fig. 65, Coch. And the
mode of ending of these nerves is very peculiar.

If we examine a section of one of the spirals of the
cochlea (Fig. 68), we see that the upper side of the cochlear
tube (that which separates it from the scala vestibuli) is
formed by a thin membrane (called the membrane of
Reissner Fig. 68 Af.R.) lined internally by simple epithe-
lium. The outer convex side of the cochlear tube, that side
by which it is firmly attached to the bony wall, is also
lined internally by simple epithelium. Neither here nor in
the membrane of Reissner do any fibres of the auditory
nerve end. But the remaining side of the tube, that
which looks towards the scala tympani, possesses on its
inner face, along the whole length of the tube, from the
bottom to the top of the spiral a very remarkable and
strangely modified epithelium ; and, along the whole length
of the tube, fibres of the auditory nerve pass into and end
among the cells of this epithehum, which is spoken of as
the o?'ga7i of Corti. (Fig. 68, O.C^

The membrane which separates the cavity of the
cochlear tube from the scala tympani, and on which the
organ of Corti is placed, is of a peculiar character, speci-
ally adapted for being thrown into vibrations, and is
called the basilar inembra7ie. The organ of Corti itself
consists of, in the first place, the so-called rods of Corti,
peculiarly shaped long bodies, which are seen in section
leaning, as it were, against each other. There is an inner
row of these and an outer row all along the spiral, each
row consisting of several (four to six) thousands of rods.
On the inside and on the outside of the rods are very
peculiar epithelial cells, also arranged into rows, each row
consisting of several thousand cells. Each of these cells
bears short hairs on its free surface, hence they are called
hair-cells, inner and outer ; and the auditory nerves pass-
ing through the lamina spiralis, reach the cochlear tube
along the whole length of the spiral, and end in filaments

VIII.]

THE COCHLEA.

227

which are lost in the organ of Corti, but are probably
connected with the hair-cells.

19. These essential parts of the organ of hearing, the
membranous labyrinth and the canalis cochlearis, are, we
have seen, lodged in chambers of the petrous part of the
temporal bone.

In the fresh state, this collection of chambers in the

t:.s.c. \

>A.S.C.

•J7..J£.

Fi'G. 6q. — Transverse Section through the Side Walls of the
Skull to show the Parts of Ear.

Co. Concha or external ear; E.M. external auditory meatus ; Ty.M. tym-
panic membrane ; hic. Mall, incus and malleus ; A.S.C., P.S.C., E.S.C.
anterior, posterior, and external semicircular canals ; Coc. cochlea ; Eu.
Eustachian tube ; 1.31. internal auditory meatus, through which the audi-
tory nerve passes to the organ of hearing.

petrous bone is perfectly closed ; but, in the dry skull, there
are two wide openings, termed feiiestrcE, or windows, on
its outer wall ; i.e.., on the side nearest the outside of the
skull. Of these fenestrae, one, termed ovalis (the oval
window), is situated in the wall of the vestibular cavity ;
the other, rotu7ida (the round window), behind and below
this, is, as we have seen, the open end of the scala tympani

Q 2

228

ELEMENTARY PHYSIOLOGY.

[less.

at the base of the spiral of the cochlea. In the fresh state,
each of these windows or fenestrae is closed by a fibrous
membrane, continuous with the periosteum of the bone.

T\ie/e?icstra rotimda is closed by membrane only ; but
fastened to the centre of the membrane of the fenestra
ovalis, so as to leave only a narrow margin, is an oval
plate of bone, part of one of the little bones to be described
shortly.

20. The outer wall of the internal ear is still far away
from the exterior of the skull. Between it and the visible

Fig. 70. — The Membrane of the Drum ok the Ear, with the small
Bones of the Ear seen from the Inner Side; and the Walls
OF THE Tympanum, with the Air-cells in the Mastoid Part of
the Temporal Bone.

The petrous part of the temporal bone containing the labyrinth is supposed
to be removed, the foot-plate of the stapes having been detached from the
fenestra ovalis.

M.C. mastoid cells ; Mall, malleus ; Inc. incus ; St. stapes ; a b, lines drawn
through the horizontal axis on which the malleus and incus turn.

opening of the ear, in fact, are placed in a straight line,
first, the drum of the ear, or tyynpamun j secondly, the
long external passage, or vieatus (Fig. 69).

The drum of the ear and the external meatus, which
together constitute the middle ear, would form one cavity,
were it not that a delicate membrane, the tympanic mem-
brane {Ty.M. Fig. 69), is tightly stretched in an oblique
direction across the passage, so as to divide the compara-
tively small cavity of the drum from the meatus.

VIII.]

THE TYMPANUM.

229

The membrane of the tympanum thus prevents any
communication, by means of the meatus, between the drum
and the external air, but such a communication is pro-
vided, though in a roundabout way, by the Eustachian
tube {Ell Fig. 69), which leads directly from the fore part
of the drum inwards to the roof of the pharj-nx, where it
opens.

Fig. 71.— a Diagram illustrative of the Relative Positions of
THE Various Parts of the Ear.

E.M. external auditory meatus; Ty.M. t>inpamc membrane; Ty. tj-m-
panum ; Mali, malleus ; Inc. incus ; Stp. stapes ; F.o. fenestra ovalis ;
F.r. fenestra rotunda ; Eu. Eustachian tube ; M.L. membraneous laby-
rinth, only one semicircular canal with its ampulla being represented ;
Sca.V., Sea. T., Sca.M., the scalae of the cochlea, which is supposed to ba
unrolled.

21. Three small bones, the auditor}- ossicles, lie in the
cavity of the tympanum. One of these is the stapes, a
small bone shaped like a stirrup. It is the foot-plate of
this bone which, as already mentioned, is firmly fastened
to the membrane of the fenestra ovalis, while its hoop
projects outwards into the t>Tnpanic cavity (Fig. 70).

Another of these bones is the malleus {Mall. Figs. 69, 70,
71), or hammer-bone, a long process, the so-called //tz«^/^.

230 ELEMENTARY PHYSIOLOGY. [LESS.

of which is fastened to the inner side of the tympanic mem-
brane (Fig. 70) ; while a ver)- much smaller process, the
sleiider process^ is fastened, as is also the body of the
malleus, to the bony wall of the tympanum by ligaments.
The rounded surface of the head of the malleus fits into a
corresponding hollowed surface in the end of a third bone,
the incus or anvil bone, thus forming a joint of a some-
what peculiar character. The incus has two processes ;
of these one, the shorter, is horizontal, and rests upon a
support afforded to it by the walls of the tympanum ; while
the other, the longer, is vertical, descends almost parallel
with the long process of the malleus, and articulates ^ with
the stapes (Figs. 70 and 71).

The three bones thus form a movable chain between
the fenestra ovalis and the tympanic membrane. The
malleus and incus are, by the peculiar joint spoken of
above, articulated together in such a manner that they
may practically be considered as forming one bone
Avhich turns upon a horizontal axis. This axis passes
through the horizontal process of the incus and the
slender process of the malleus, and its ends rest in the
walls of the tympanum. Its general direction is repre-
sented by the line ^ ^ in Fig. 70, or by a line perpendicular
to the plane of the paper, passing through the head of the
malleus in Fig. 71.

The two bones may be roughly compared to two spokes
of a wheel, of which the axle is represented by the axis just
described ; it should be added, however, that one spoke,
the incus, is shorter than the other, and that the movement
of the two spokes is limited to a ver}' small arc of a circle.

When the membrane of the drum, thrown into vibration
by some sound, moves inwards and outwards in its vibra-
tions, it necessarily carries with it, in each inward and
outward movement, the handle of the malleus which is
attached to it. But with each inward and outward move-
ment of the handle of the malleus, the long process of the
incus also moves inward and outward, carrj-ing with it the
stapes which is attached to its end. Hence each vibration,

A minute bone, the os orbiculare, inter\'enes between the end of the pro-
cess of the incus and the stapes, so that the stapes is in reality articulated
with the OS orbiculare, which in turn is fastened to the process of the incus.
For simplicity's sake, mention of this is omitted above.

VIII.] THE AUDITORY OSSICLES. 23I

each inward thrust, and each outward or backward return
of the membrane of the drum, produces by means of the
chain of ossicles a corresponding vibration of the mem-
brane of the fenestra ovahs to which the stapes is
attached ; ' but the vibrations of this membrane are in
turn communicated to the perilymph of the labyrinth and
cochlea. Thus by means of the chain of ossicles, and the
membranes to which these are attached at each end, the
aerial vibrations passing down the meatus are transformed
into corresponding vibrations of the fluids of the inner
ear. The vibrations of the perilymph passing up the
scala vestibuli, and down the scala tympani, reach at last
the membrane covering the fenestra rotunda and throw
this into vibration ; and as a matter of fact it has been
observed that when the membrane of the fenestra ovalis
moves inward, that of the fenestra rotunda move's out-
wards, and vice versa.

The vibrations of the perilymph thus produced will
affect the endolymph, and this the hairs, and so the
auditory epithelium of the labyrinth and cochlea ; by
which, finally, the auditory nerves will be excited.

22. The characters of the vibration of a membrane, and
the readiness with which it takes up or responds to, aerial
vibrations reaching it, are largely modified by its degree
of tension ; the membrane acts differently when it is tightly
stretched from what it does when it is loose. Now, within
the cavity of the tympanum are two small, but relatively
strong muscles. One, called the stapedius, -^^lssq^s from the
floor of the tympanum to the foot of the stapes and the
orbicular bone, the other, the tefisoi'Jynipani, from the front
wall of the drum to the malleus. Each of the muscles
when it contracts tightens the membrane to which it is
thus indirectly attached, the tensor tympani, the membrane
of the drum, and the stapedius, the membrane of the
fenestra ovalis. The effect of thus tightening the mem-
brane is probably to restrict the vibrations of the mem-
brane, at least as far as concerns grave, or low-pitched

' Owing to certain characters in the attachment of the stapes to the
membrane of the fenestra ovalis on the one hand, and to the os orbiculare on
the other, the movements of the foot of the stapes in the fenestra ovalis are
somewhat peculiar ; but the details of these as well as the functions of the
peculiar articulation of the incus with the malleus, have, for simplicity's sake,
been omitted.

232 ELEMENTARY PHYSIOLOGY. [less.

sounds ; but the complete action of these muscles is too
intricate to be dwelt on here.

23. The outer extremity of the external meatus is sur-
rounded by the concha or external ear {Co. Fig. 69), a
broad, peculiarly-shaped, and for the most part cartila-
ginous plate, the general plane of which is at right angles
with that of the axis of the auditor)^ opening. The concha
can be moved by most animals and by some human beings
in various directions by means of muscles, which pass to
it from the side of the head.

24. The manner in which the complex apparatus now
described intermediates between the physical agent, which
is the primary cause of the sensation of sound, and the ner-
vous expansion, the affection of which alone can excite
that sensation, must next be considered.

All bodies which produce sound are in a state of vibra-
tion, and they communicate the vibrations of their own
substance to the air with which they are in contact and
thus throw that air into waves, just as a stick waved
backwards and forwards in water throws the water into
waves.

The aerial waves, produced by the vibrations of sono-
rous bodies, in part enter the external auditory passage,
and in part strike upon the concha of the external ear and
the outer surface of the head. It maybe that some of the
latter impulses are transmitted through the solid struc-
ture of the skull to the organ of hearing ; but before they
reach it they must, under ordinary circumstances, have
become so scanty and weak, that they may be left out of
consideration.

The aerial waves which enter the meatus all impinge
upon the membrane of the drum and set it vibrating,
stretched membranes, especially such as have the form
and characters of the tympanic membrane, taking up
vibrations from the air with great readiness.

25. The vibrations thus set up in the membrane of the
tympanum are communicated, in part, to the air contained
in the drum of the ear, and, in part, to the malleus, and
thence to the other auditor)' ossicles.

The vibrations communicated to the air of the drum
impinge upon the inner wall of the tympanum, on the

vni.] THE FUNCTION OF THE OSSICLES. 233

greater part of which, from its density, they can. produce
very httle effect. Where this wall is formed by the
membrane of i\\Q fe7testra rotujida the communication of
motion must necessarily be greater. All these vibrations,
however, may probably be neglected.

The vibrations which are communicated to the malleus
and the chain of ossicles may be of two kinds : vibrations
of the particles of the bones, and vibrations of the bones
as a whole. If a beam of wood, freely suspended, be very
gently scratched with a pin, its particles will be thrown
into a state of vibration, as will be evidenced by the sound
given out, but the beam itself will not be visibly moved.
Again, if a strong wind blow against the beam, it will
swing bodily, without any vibrations of its particles among
themselves. On the other hand, if the beam be sharply
struck wuth a hammer, it will not only give out a sound,
showing that its particles are vibrating, but it will also
swing, from the impulse given to its whole mass.

Under the last-mentioned circumstances, a blind man
standing near the beam would be conscious of nothing but
the sound, the product of molecular vibration, or invisible
oscillation of the particles of the beam ; while a deaf man
in the same position would be aware of nothing but the
visible oscillation of the beam as a whole.

26. Thus, to return to the chain of auditory ossicles,
while it may be supposed that, when the membrane of the
drum vibrates, these may be set vibrating both as a whole
and in their particles, the question arises whether it is
the large vibrations, or the minute ones, which make
themselves obvious to the auditory nerve, which is in the
position of our deaf, or blind, man.

The evidence is distinctly in favour of the conclusion,
that it is the vibrations of the bones, as a whole, which are
the chief agents in transmitting the impulses of the aerial
waves.

For, in the first place, the disposition of the bones and
the mode of their articulation are very much against the
transmission of molecular vibrations through their sub-
stance, but, on the other hand, are extremely favour-
able to their vibration eii masse. The long processes of
the malleus and incus swing, like a pendulum, upon the
axis furnished by the short processes of these bones ; while

234 ELEMENTARY PHYSIOLOGY. [less

the mode of connection of the incus with the stapes, and
of the latter with the membrane of the fenestra ovahs,
allows the foot plate of that bone free play, inwards and
outwards. In the second place, the total length of the
chain of ossicles is very small compared with the length
of the waves of audible sounds, and physical considera-
tions teach us that in a like thin rod, similarly capable
of swinging e7i masse, the minute molecular vibrations
would be inappreciable. Thirdly, direct experiments,
such as attaching to the stapes of a dissected ear, a
light style, the movements of which are recorded on a
travelling smoked glass plate or in some other way, show
that the chain of ossicles does actually vibrate as a whole,
and at the same rate as the membrane of the drum,
when aerial vibrations strike upon the latter.

27. Thus, there is reason to believe that when the tym-
panic membrane is set vibrating, it causes the process of
the malleus, which is fixed to it, to swing at the same rate ;
the head of the malleus consequently turns through a small
arc on its pivot, the slender process. But, as stated in
§ 21, the turning of the head of the malleus involves the
simultaneous turning of the head of the incus upon its
pivot, the short process. In consequence the long pro-
cess of the incus also swings at the same rate. The length
of the long process of the incus, measured from the
axis, on which the two bones turn, is less than that of
the handle of the malleus ; hence the end of it moves
through a smaller space. The arc through which it moves
has been estimated as being equal to about two-thirds of
that described by the handle of the malleus. The extent
of the push is thereby somewhat diminished, but the force
of the push is proportionately increased ; in so confined a
space this change is advantageous. The long process of
the incus, however, is so fixed to the stapes, and the
stapes so attached to the membrane of the fenestra ovalis,
that the incus cannot vibrate without throwing into vibra-
tions, to a corresponding extent and at the same rate, the
membrane of the fenestra ovalis.^ But every vibration,
every pull and push, imparts a corresponding set of
shakes to the perilymph, which fills the bony labyrinth
and cochlea, external to the membranous labyrinth and

I See foot-note, p. 231.

viii.i AUDITORY SENSATIONS. 235

canalis cochlearis. These shakes are communicated to
the endolymph in the latter chambers, and, by the help
of the modified auditory epithelium described above,
stimulate the delicate endings of the vestibular and
cochlear divisions of the auditory nerve.

28. We do not at present know what kind of changes
the vibrations of the endolymph give rise to in the epi-
thelial cells of the macuke of the utriculus and sacculus, of
the crests of the ampulhe, and of the organ of Corti ; nor
do we at present know the exact way in which the changes
thus set up in these epithelial cells are able to excite the
terminal filaments of the auditory nerve. But there can
be no doubt of the fact that the elaborate apparatus of
the cochlea and the simpler apparatus of the labyrinth
are able to translate, so to speak, the sonorous vibrations
which reach them into stimulations of nerve fibres, the
molecular changes of which are transmitted along the
auditory nerve as auditor)^ nerv^ous impulses. Passing
along the auditory nerve, these molecular changes, these
nervous impulses, reach certain parts of the brain, the
exact situation of which is at present a matter of conjec-
ture, and there in turn setup those molecular disturbances
of ner\'ous matter which form the immediate cause of the
states of feeling called " sounds." Thus the auditory
nerve may be said, and a similar statement may be made
in the case of the other nerv^es of special sensations, to
be provided with two " end-organs." There is the peri-
pheral end-organ (the apparatus of the cochlea and
labyrinth), by which the physical agent is enabled to
excite the sensory nerve-fibres ; and there is the ce7itral
end-organ^ in the brain, in which the ner\'ous impulses of
the sensory nerve excite the special state of feeling which
we call the special sensation. The central end-organ of
hearing is often spoken of as the auditor}- sensorium.

Between the sounding body and the actually hearing a
sound there is a chain of events of different kinds. There
are the vibrations started by the sounding body, and
passing through the air, the tympanum, the perilymph,
and the endolymph ; these are all of one order. Then
there are the changes in the peripheral end-organ, in the
apparatus of the cochlea and labyrinth ; these are of
another order. Then follow the molecular disturbances

236 ELEMENTARY PHYSIOLOGY. [less.

travelling along the auditory nerve ; these are of still
another order. Lastly, there are the changes in the
central end-organ, in the brain ; these, though resembling
the preceding in so far as they are changes of nervous
matter, are yet of still another order, and probably com-
prise in themselves a whole series of events, the conse-
quence of the last of which is the sensation of sound.

29. The differences between the functions of the mem-
branous labyrinth (to which the vestibular nerve is dis-
tributed) and those of the cochlea are not quite certainly
made out, but the following view has been suggested : —

Ever}' sound consists, as we have seen, of vibrations.
Sometimes the vibrations are repeated with great regu-
larity ; and sounds, in which the regular recurrence of
the same ^■ibrations is conspicuous, are called " musical
sounds.'"' Sometimes no regular repetition of vibrations
can be recognised ; the sound consists of vibrations,
few of which are like each other, and which fall irregu-
larly on the ear ; such sounds are called '' noises."

When we listen to musical sounds, each set of regularly
repeated vibrations generates in the central end-organ a
particular kind of sensation which we call a to7ie ; and
the simultaneous or successive production of different
tone-sensations gives rise in us to the feelings which we
speak of as those of harmony or melody.

When we listen to a noise the vibrations generate
sensations which are of a certain intensity, according to
which we call the noise slight or great, low or loud, and
which also have certain characters by which we recognise
the kind of noise ; but the sensations have not the
qualities of tone-sensations, and do not give rise to feelings
of melody or harmony.

And it has been suggested that the arrangements of
the cochlea are such that musical sounds are enabled to
excite the cochlear ner\-e, and to generate in the central
end-organ connected with it sensations of tone ; while
the arrangements of the labyrinth and the central end-
organ of the vestibular nerve are such as to be readily
affected by noises.

Such a view is not without difficulties ; but the following
considerations render it probable that the cochlea at least
is adapted for the appreciation of musical sounds.

VIII. J MUSICAL SOUNDS. 237

30. A pure musical sound consists of a series of vibra-
tions repeated with exact regularity, the number of
vibrations occurring in a given time, e.g. in a second,
determining what is called the pitch of the " note." But
ordinary musical sounds are, for the most part, not simple,
consisting of one set of vibrations, but compound, con-
sisting of several sets of vibrations occurring together ;
in these musicians distinguish one set, called the funda-
me7ital tone, and other sets, varying in intensity or
loudness, called overtones.

A tuning-fork, when set vibrating, vibrates with a
given rapidity ; and the note given out is determined by
the rapidity of the vibration, by the number of vibrations
repeated, for instance, in a second ; hence every tuning-fork
has its own proper note. Now, a tuning-fork will be set
vibrating if its own particular note be sounded in its neigh-
bourhood, but not if other notes be sounded. Hence,
when a pure musical note is sounded close to a number of
tuning-forks of different pitch, only that tuning-fork the
pitch of which is the same as that of the note sounded is
set vibrating ; the others remain motionless. When an
ordinary musical sound, such as a note sung by the human
voice, is produced among such a group of tuning-forks,
several are set vibrating ; one of these corresponds to the
fundamental tone, and the others to the various overtones
of the sound. Similarly, if the top of a piano be lifted up
or removed, and any one sings into the wires with sufficient
loudness, a note, such as the tenor c, a number of the wires
will be set vibrating, one corresponding to the fundamental
tone, and the others to the overtones.

If we were to imagine an immense number of tuning-
forks, each vibrating at different periods, so arranged that
each fork, when vibrating, in some way or other stimulated
or excited a minute delicate nerve-filament attached to it,
it is obvious that a musical sound uttered near these
tuning-forks would set a certain number of them into
vibration, some more forcibly than others, and that in
consequence a certain number, and a certain number only,
of the delicate nerve filaments would be excited, and that
to various degrees ; and thus a particular series of nervous
impulses, the counterpart as it were of the musical sound

238 ELEMENTARY PHYSIOLOGY. [less.

with its funaamental tone and overtones, would be trans-
mitted along the nerve filaments to the brain.

And it is suggested that the basilar membrane of
the cochlea, consisting as it does of thousands of fibres
stretching across from the inside to the outside (from left
to right in Fig. 68), with its thousands of epithelial cells
and rods of Corti lying upon it, represents, as it were, an
assemblage of thousands of tuning-forks of various rates
of vibration, with a separate nerve filament attached to
each. So that, when a number of vibrations of different
periods, such as constitutes an ordinary musical sound, are
transmitted by the tympanum to the cochlea, these as
they sweep along the canalis cochlearis throw into sym-
pathetic movement those parts, and those parts only, of
the basilar membrane with their overlying epithelium and
rods of Corti, whose periods of vibration correspond to their
o^\^l vibrations, and thus excite certain nerve filaments,
and these only. It is this excitement of a group of nerve
filaments, some more intensely than others, which reach-
ing the brain gives rise to the sensation which we associate
with the particular musical sound.

As has been already stated, we know ven,' little definitely
about the position in the brain of, and still less about the
nature of, the auditory sensorium or central end-organ of
the auditor)^ nerve ; but it may be conceived that each
filament of the cochlear ners'e is connected with a par-
ticular portion of the nervous matter of the central end-
organ, in such a way that the molecular movements of one
of these particular portions of nervous matter, brought
about by a molecular disturbance reaching it through its
appropriate filament, produces a psychical effect of one
kind only, more or less intense it may be, but still always
of one kind. If this be so, each cochlear fibre or filament
may be considered as being provided with two end-organs :
one, peripheral, in the organ of Corti, capable of being set
in motion by vibrations of one quality only ; the other,
central, in the brain, capable of producing a psychical effect
of one quality only. It does not follow, however, that we
are distinctly and separately conscious of the nervous dis-
turbance in each central end-organ, it does not follow
that we have as many distinct and separate kinds of

VIII.] FUNCTIONS OF THE COCHLEA. 239

conscious sensation as there are peripheral and central
end-organs, though how many such distinct kinds of
sensation we may have we do not know. Just as the
peripheral mechanism sifts out the several vibrations of
which a musical sound is composed, and transmits them
separately, so, by a reverse operation, the central mech-
anism probably pieces together the nervous disturbances
of a number of central end-organs, and thus produces a
sensation whose characters are determined by a com-
bination of the nervous disturbances taking place in each
end-organ.

Some such a view is indeed exceedingly probable ; but
it must be remembered that we do not at present at all
understand the exact mechanism by which each particular
vibration excites its corresponding nen,-e filament. The
nerve filaments appear to end in the epithelial cells bear-
ing short hairs, which lie on each side of the rods of Corti ;
and we may therefore conclude that these "hair-cells"
have some share in producing the effect. But the whole
matter is at present very obscure ; the functions of the
rods of Corti are particularly difficult to understand ; for
these do not seem in any way connected with the nerve
filaments, and their movements can only affect the latter
by influencing in some way the hair-cells.

31. The fibres of the cochlear nerve, or their endings in
the brain itself, may be excited by internal causes, such as
the varying pressure of the blood and the like : and in
some persons such internal influences do give rise to
veritable musical spectra, sometimes of a ver}'- intense
character. But, for the appreciation of music produced
external to us, we depend upon the organ of Corti being
in some way or other affected by the vibrations of the
fluids in the cochlea.

32. It has already been explained that the stapedius and
te?isor tyf??panim.ViSc\es, are competent to tighten the mem-
brane of the fenestra ovalis and that of the tympanum,
and it is probable that they come into action when the
sonorous impulses are too violent, and would produce too
extensive vibrations of these membranes. They may
therefore be of use in moderating the effect of intense
sound, in much the same way that, as we shall find, the

240 ELEMENTARY PHYSIOLOGY. [less.

contraction of the circular fibres of the iris tends to
moderate the effect of intense Hght in the eye ; they may
however, have other purposes.

The function of the Eustachian tube is, probably, to
keep the air in the tympanum, or on the inner side of the
tympanic membrane, of about the same tension as that on
the outer side, which could not always be the case if the
tympanum were a closed cavity.

IX. 1 THE EYE. 241

LESSON IX.

THE ORGAN OF SIGHT.

1. In Studying the organ of the sense of sight, the eye,
it is needful to become acquainted, firstly, with the struc-
ture and properties of the sensory expansion in which the
optic nerve, or nerve of sight, terminates ; secondly, with
the physical agent of the sensation ; thirdly, with the
intermediate apparatus by which the physical agent is
assisted in acting upon the nervous expansion.

The ball, or globe, of the eye is a globular body, mov-
ing freely in a chamber, the orbit^ which is furnished to it
by the skull. The optic nerve, the root of which is in
the brain, leaves the skull by a hole at the back of the
orbit, and enters the back of the globe of the eye, not in
the middle, but on the inner, or nasal, side of the centre.
Having pierced the wall of the globe, it spreads out into
a very delicate membrane, varying in thickness from
sVth of an inch to less than half that amount, which lines
the hinder two-thirds of the globe, and is termed the
retina. This retina is the only organ connected with
sensory nervous fibres which can be affected, by any
agent, in such a manner as to give rise to the sensation
of light.

2. If the globe of the eye be cut in two, transversely, co
as to divide it into an anterior and a posterior half, the
retina will be seen lining the whole of the concave wall of
the posterior half as a membrane of great delicacy, and,
for the most part, of even texture and smooth surface.
But almost exactly opposite the middle of the posterior

^42

ELEMENTARY PHYSIOLOGY. [less,

td^

.mm&

Fig. 72. — Diagrammatic Views of the Nervous (A) and the Con-
nective (B) Elements of the Retina, supposed to be separated

FROM one another.

A, the nervous structures — b, tlie rods ; c, the cones ; b' cf, the granules or
nuclei of the outer layer, with which these are connected ; d d', inter-
woven very delicate nervous fibres, from which fine nervous filaments,
bearing the inner granules or nuclei, f/', proceed towards the inner sur-
face ; gg', the continuation of these fine nerves, which become convoluted
and interwoven with the processes of the nerve cells h h' ; / /, the expan-
sion of the fibres of the optic nerve. B, the connective tissue — a a,
external limiting membrane ; e e, radial fibres passing to the internal
limiting membrane; ef e' , nuclei; d d, the intergranular layer; g g, the
uoiecular layer ; /, the inner limiting membrane.

(Magnified about 250 diameters.)

IX.1

THE RETINA.

243

wall, it presents a slight circular depression of a yellowish
hue, the macula lutea^ or yellow spot (Fig. 73, ni.l. ; Fig. 76,
8"), — not easily seen, however, unless the eye be perfectly
fresh, — and, at some distance from this, towards the inner,
or nasal, side of the ball, is a radiating appearance, pro-
duced by the entrance of the optic nerve and the spreading
out of its fibres into the retina.

Fig. 73. — The Eyeball divided transversely in the middle line,

AND viewed from THE FrONT.

J, sclerotic ; ch^ choroid, seen in section only.

r, the cut edges of the retina ; f .v, vessels of the retina springing from o,

the optic nerve or blind spot ; in. I, the yellow spot, the darker spot in

its middle being the fovea centralis.

3. A very thin vertical slice of the retina, in any region
except the yellow spot and the entrance of the optic
nerve, may be resolved into the structures represented
separately in Fig. 72. The one of these (A) occupies
the whole thickness of the section, and comprises its
essential, or nervous, elements. The outer ^ fourth, or

I In the following account of the retina, the parts are described in relation
to the eyeball. Thus, that surface of the retina which touches the vitreous
humour, and so is nearer the centre of the eyeball, is called the inner
surface ; and that surface which touches the choroid coat is called the oiiter
surface. And so with the structures between these two surfaces ; that which
is called inner is nearer the vitreous humour, and that which is called outer

R 2

244 ELEMENTARY PHYSIOLOGY. [less.

rather less, of the thickness of these consists of a vast
multitude of minute, either rod-like, or conical bodies,
ranged side by side, perpendicularly to the plane of the
retina. This is the layer of rods a?td cones {b c). From the
front ends or bases of the rods and cones very delicate
fibres pass, and in each is developed a granule-like or
nucleus-like body {b' c'), which forms a part of what has
been termed the outer layer of gra?iules, or outer nuclear
layer. It is probable that these fibres next pass into
and indeed form the close meshwork of very delicate
nervous fibres which is seen at d d' (Fig. 72, A). From
the inner surface of this meshwork other fibres proceed,
containing a second set of granules or nuclei, which forms
the i7iner granular layer., or itiner nuclear layer {ff).
Inside this layer is a stratum of convoluted fine nerv^ous
fibres {ji^g') — and inside this again are numerous nerve-
cells {h //'). Processes of these nerve-cells extend, on the
one hand, into the layer of convoluted nerve-fibres ; and
on the other are probably continuous with the stratum of
fibres of the optic nen-e (z).

These delicate nervous structures are supported by a
sort of framework of connective tissue of a peculiar kind
(B), which extends from an in?ier or anterior limiting
jnenibrane {I), which bounds the retina and is in contact
with the vitreous humour, to an outer or posterior limiting
membrane, which lies at the inner ends, or bases, of the
rods and cones near the level of b' c' in A. Thus the
framework falls short of the nervous substance of the
retina, and the rods and cones lie altogether outside of it,
wholly unsupported by any connective tissue. They are,
however, as we shall see, imbedded in the layer of pigment
on which the retina rests (§ 16}.

The fibres of the optic nerve spread out between the
limiting membrane (/) and the nen-e-cells (//'), and the
vessels which enter along with the optic nerve ramify
between the two limiting membranes, most of them
running between the inner limiting membrane and the
inner nuclear layer iff). Thus, not only the nervous
fibres, but the vessels, are placed altogether in front of the
rods and cones.

is nearer the choroid coat. Sometimes anterior, or front, is used instead of
inner, aiid/c7i/^r/V»r' instead of outer.

IX.]

THE RETINA.

245

Fig. 74. — A Diagrammatic Section of the Maclla Lltea, or
Yellow Spot.

a a, the pigment of the choroid ', b c, rods and cones ; d d, outer granular or
nuclear layer ; yy, inner granular or nuclear layer ; gg; molecular layer ;
hh, layer of nerve cells ; i i, fibres of the optic nerve.

(Magnified about 60 diameters.)

246 ELEMENTARY PHYSIOLOGY [less.

At the entrance of the optic nerve itself, the ner\-ous
fibres predominate, and the rods and cones are absent.
In the yellow spot, on the contrar)', the cones are abun-
dant and close set, becoming at the same time longer and
more slender, while rods are scanty, and are found only
towards its margin. The layer of fibres of the optic
nerve disappears, and all the other layers, except that of
the cones, becom.e extremely thin in the centre of the
macula hitea (Fig. 74).

4. The most notable property of the retina is its power
of converting the vibrations of ether, which constitute the
physical basis of light, into a stimulus to the fibres of the
optic nerve. The central ends of these fibres are con-
nected with certain parts of the brain which constitute
the visual sefisoriinn, just as other parts, as we have seen,
constitute the auditor}- sensorium. The molecular dis-
turbances set up in the fibres of the optic nerve are
transmitted to the substance of the visual sensorium, and
produce changes in the latter, giving rise to the state of
feeling which we call a sensation of light.

The sensation of light, it must be understood, is the
work of the visual sensorium, not of the retina ; for, if
an eye be destroyed, pinching, galvanizing, or otherwise
irritating the optic nerve, will still excite the sensation of
light, because it throws the fibres of the optic nerve into
activity ; and their activity, however produced, brings
about in the visual sensorium certain changes which give
rise to the sensation of light.

Light, falling directly on the optic nerve, does not
excite it ; the fibres of the optic nerve, in themselves, are
as blind as any other part of the body. But just as the
peculiar hair cells of the labyrinth, and the organ of Corti
of the cochlea, are contrivances for converting the delicate
vibrations of the perilymph and endolymph into impulses
which can excite the auditory nerves, so the structures in
the retina appear to be adapted to convert the infinitely
more delicate pulses of the luminiferous ether into stimuli
of the fibres of the optic nerve.

5. The sensibility of the different parts of the retina to
light varies very greatly. The point of entrance of the
optic nerve is absolutely blind, as may be proved by a
vtry simple experiment. Close the left eye, and look

IX.J

THE BLIND SPOT.

steadily with the right at the cross on the page, held at
ten or twelve inches' distance.

The black dot will be seen quite plainly, as well as the
cross. Now, move the book slowly towards the eye, which
must be kept steadily fixed upon the cross ; at a certain
point the dot will disappear, but, as the book is brought
still closer, it will come into view again. It results from
optical principles that, in the first position of the book,
the image of the dot falls between that of the cross
(which throughout lies upon the yellow, spot) and the

OoJ'^C

Fig. 75.— Pigment Cells from the Choroid Coat.

A, branched pigment cells from the deep layer.

B, pigment epithelium, «, seen in face; b, seen in profile; c, pigment
granules.

entrance of the optic nerve : while, in the second position,
it falls on the entrance of the optic nen-e itself; and, in
the third, inside that point. So long as the image of the
spot rests upon the entrance of the optic nen-e, it is not
perceived, and hence this region of the retina is called
the blind spot. The experiment proves that the vibrations
of the ether are not able to excite the fibres of the optic
nerve itself.

6. The impression made by light upon the retina not
only remains during the whole period of the direct action
of the light, but has ?. certain duration of its own, how-
ever short the time during which the light itself lasts. A

248 ELEMENTARY PHYSIOLOGY [less.

llash of lightning is, practically, instantaneous, but the
sensation of light produced by that flash endures for an
appreciable period. It is found, in fact, that a luminous
impression lasts for about one-eighth of a second ;
whence it follows, that if any two luminous impressions
are separated by a less interval, they are not distinguished
from one another.

For this reason a Catherine-wheel," or a lighted stick
turned round very rapidly by the hand, appears as a circle
of fire ; and the spokes of a coach wheel at speed are not
separately visible, but only appear as a sort of opacit)-, or
film, within the tire of the wheel.

7. The excitability of the retina is readily exhausted.
Thus, looking at a bright light rapidly renders the part of
the retina on which the light falls, insensible ; and on
looking from the bright light towards a moderately-lighted
surface, a dark spot, arising from a temporary blindness
of the retina in this part, appears in the field of view. If
the bright light be of one colour, the part of the retina on
which it falls becomes insensible to rays of that colour,
but not to the other rays of the spectrum. This is the
explanation of the appearance of what are called comple-
mentary colours. For example, if a bright red wafer be
stuck upon a sheet of white paper, and steadih; looked at
for some time with one eye, when the eye is turned aside
to the white paper a greenish spot will appear, of about
the size and shape of the wafer. The red image has, in
fact, fatigued the part of the retina on which it fell for red
light, but has left it sensitive to the remaining coloured
rays of which white light is composed. But we know that
if from the variously coloured rays which make up the
spectrum of white light we take away all the red rays, the
remaining rays together make up a sort of green. So that,
when white light falls upon this part, the red rays in the
white light having no effect, the result of the operation of
the others is a greenish hue. If the wafer be green^ the
complementary image, as it is called, is red.

8. Most people agree very closely as to differences
between different colours and different parts of the
spectrum. But there are exceptions. Thus a certain
number of persons see ver\' little difference between the
colour which most people call red, and that which most

IX.] COLOUR BLINDNESS.. 249

people call green Such colour-blind persons are unable
to distinguish between the leaves of a chern--tree and its
fruit by the colour of the two ; they are only aware of a
difference of shape between the two. Cases of this " red-
blindness " or " red-green "" blindness are not uncommon : but
another form of colour blindness in which blue and yellow
cannot be distinguished from each other is much more
rare ; and though it has been asserted that persons have
been found, who were wholly colour blind, i.e. to whom
all colours were mere shades of one tint, such cases are
not beyond doubt.

This peculiarity of colour-blindness is simply un-
fortunate for most people, but it may be dangerous if
unknowingly possessed by railway guards or sailors. It
probably arises either from a defect in the retina, which
renders that organ unable to respond to different kinds
of luminous vibrations, and consequently insensible to
red, yellow, or other rays, as the case may be ; or the
fault may lie in the visual sensorium itself.

9. The sensation of light may be excited by other
causes than the impact of the vibrations of the lumi-
niferous ether upon the retina. Thus, an electric shock
sent through the eye or through the optic nen-e gives rise
to the appearance of a flash of light : and pressure on any
part of the retina produces a luminous image, which lasts
as long as the pressure, and is called 2. phosphene. If the
point of the finger be pressed upon the outer side of the
ball of the eye, the eyes being shut, a luminous image—
which, in my own case, is dark in the centre, with a bright
ring at the circumference (or, as Xewton described it, like
the " eye *' in a peacock's tail-feather) — is seen ; and this
image lasts as long as the pressure is continued. Most
persons, again, have experienced the remarkable display
of subjective fireworks which follows a hea\y blow about
the region of the eyes, produced by a fall from a horse,
or by other methods well known to English youth.

It is doubtful, however, whether these ettects of pressure,
or shock, really arise from the excitation of the retina
proper, or whether they are not rather the result of the
violence done to the fibres of the optic nerve apart from
the retina.

10. The last paragraph raises a distinction between

250 ELEMENTARY PHYSIOLOGY. [less.

the " fibres of the optic nerve " and the " retina " which
may not have been anticipated, but which is of much
importance.

We have seen that the fibres of the optic nerve ramify
in the inner fourth of the thickness of the retina, while
the layer of rods and cones forms its outer fourth. The
light, therefore, must fall first upon the fibres of the optic
nerve, and, only after traversing them, can it reach the
rods and cones. Consequently, if the fibrillse of the optic
nerve themselves are capable of being affected by light,
the rods and cones can only be some sort of supple-
mentary optical apparatus. But, in fact, it is the rods and
cones which are affected by light, while the fibres of the
optic nene are themselves insensible to it. The evidence
on which this statement rests is : —

a. The blind spot is full of nervous fibres, but has no
cones or rods.

d. The yellow spot, where the most acute vision is
situated, is full of close-set cones, but has no nerve
fibres.

c. If one goes into a dark room with a single small
bright candle, and, looking towards a dark wall, moves
the light up and down, close to the outer side of one eye,
so as to allow the light to fall very obliquely into the eye,
one of what are called Pu?'/ciiijes figures is seen. This
is a vision of a series of diverging, branched, dark, some-
times reddish, lines on an illuminated field, and in the
interspace of two of these lines is a sort of cup-shaped
disk. The branched lines are the images of shadows thrown
by the retinal blood-vessels, and the disk is that of the
shadow thrown by the edge of the yellow spot. As the
candle is moved up and down, the lines shift their posi-
tion, as shadows do when the light which throws them
changes its place.

Now, as the light falls on the inner face of the retina,
and the images of the vessels to which it gives rise shift
their position as it moves, whatever constitutes the end-
organ, through which light stimulates the fibres of the
optic nerve, must needs lie on the other, or outer, side of
the vessels. But the fibres of the optic nerve lie among
the vessels, and the only retinal structures which lie out-
side them are the nuclear layers and the rods and cones,

IX.] CONDITIONS OF DISTINCT VISION. 251

d. Just as, in the skin, there is a hmit of distance
within which two points give only one impression, so there
is a minimum distance by which two points of Hght falhng
on the retina must be separated in order to appear as
two. And this distance corresponds pretty well with the
diameter of a cone.

II. The impact of the ethereal vibrations upon the
sensory expansion, or essential part of the visual appa-
ratus alone, is sufficient to give rise to all those feelifigs^
which we terifi sensations of light and of colour^ and
further to that feeling of outness which accompanies all
visual sensation. But, if the retina had a simple trans-
parent covering, the vibrations radiating from any number
of distinct luminous points in the external world would
affect all parts of it equally, and therefore the feeling
aroused would be that of a generally diffused luminosity.
There would be no separate feeling of light for each
separate radiating point, and hence no correspondence
between the visual sensations and the radiating points
which aroused them.

It is obvious that, in order to produce this correspond-
ence, or, in other words, to have distinct vision, the essential
condition is, that distinct luminous points in the external
world shall be represented by distinct feelings of light.
And since, in order to produce these distinct feelings,
vibrations must impinge on separate rods or cones, or
at least on separate parts of the retina, it follows that, for
the production of distinct vision, some apparatus must
be interposed between the retina and the external world,
by the action of which, distinct luminous points in the
latter shall be represented by corresponding points of
light on the retina.

In the eye of man and of the higher animals, this acces-
sory apparatus of vision is represented by structures which,
taken together, act as a biconvex lens, composed of sub-
stances which have a much greater refractive power than
the air by which the eye is surrounded ; and which throw
upon the retina luminous points, which correspond in
number, and in position relatively to one another, with
those luminous points in the external world from which
ethereal vibrations proceed towards the eye. The lumin-
ous points thus thrown upon the retina form a picture

252 ELEMENTARY PHYSIOLOGY. [less.

of the external world — a picture being nothing but lights
and shadows, or colours, arranged in such a way as to
correspond with the disposition of the luminous parts of
the object represented, and with the qualities of the light
which proceeds from them.

12. That a biconvex lens is competent to produce a
picture of the external world on a properly arranged
screen is a fact of which every one can assure himself by
simple experiments. An ordinary spectacle glass is a
transparent body denser than the air, and Convex on both
sides. If this lens be held at a certain distance from
a screen or wall in a dark room, and a lighted candle be
placed on the opposite side of it, it will be easy to adjust
the distances of candle, lens, and wall, in such a manner
that an image of the flame of the candle, upside down,
shall be thrown upon the wall.

The spot on which the image is formed is called a _/^a^j.
If the candle be now brought nearer to the lens, the image
on the wall will enlarge, and grow blurred and dim, but
it may be restored to brightness and definition by moving
the lens further from the wall. But if, when the new
adjustment has taken place, the candle be moved away
from the lens, the image will again become confused, and
to restore its clearness, the lens will have to be brought
nearer the wall.

Thus a convex lens forms a distinct picture of luminous
objects, but only at the focus on the side of the lens
opposite to the object ; and that focus is nearer when the
object is distant, and further off when it is near.

1 3. Suppose, however, that, leaving the candle unmoved,
a lens with more convex surfaces is substituted for the
first, the image will be blurred, and the lens will have to
be moved nearer the wall to give it definition. If, on
the other hand, a lens with less convex surfaces is sub-
stituted for the first, it must be moved further from the
wall to attain the same end.

In other words, other things being alike, the more con-
vex the lens the nearer its focus ; the less convex, the
further off its focus.

If the lens were made of some extensible, elastic sub-
stance, like india-rubber, pulling it at the circumference
would render it flatter, and thereby lengthen its focus ;

IX.] ACCESSORY PARTS OF THE EYE. 253

while, when let go again, it would become more convex,
and of shorter focus.

Any material more refractive than the medium in which
it is placed, if it have a convex surface, causes the rays of
light which pass through the less refractive medium to
that surface to converge towards a focus. If a watch-glass
be fitted into one side of a box, and the box be then filled
with water, a candle may be placed at such a distance
outside the watch-glass that an image of its flame shall
fall on the opposite wall of the box. If, under these cir-
cumstances, a doubly convex lens of glass were introduced
into the water in the path of the rays, it would act (though
less powerfully than if it were in air) in bringing the rays
more quickly to a focus, because glass refracts light more
strongly than water does.

A camera obsciira is a box, into one side of which a lens
is fitted, so as to be able to slide backwards and forwards,
and thus throw on the screen at the back of the box dis-
tinct images of bodies at various distances off. Hence
the arrangement just described might be termed a water
camera.

14. The accessory organs, by means of which the
physical agent of vision, light, is enabled to act upon the
expansion of the optic nerve, comprise three kinds of
apparatus : {a) a "water camera," the eyeball ; {b) muscles
for moving the eyeball ; {c) organs for protecting the
eyeball, viz. the eyelids, with their lashes, glands, and
muscles ; the conjunctiva ; and the lachrymal gland and
its ducts.

The eyeball is composed, in the first place, of a tough,
firm, spheroidal case consisting of fibrous or connective
tissue, the greater part of which is white and opaque, and
is called the sclerotic (Fig. 76, 2). In front, however,
this fibrous capsule of the eye, though it does not change
its essential character, becomes transparent, and receives
the name of the cornea (Fig. 76, i). The corneal por-
tion of the case of the eyeball is more convex than the
sclerotic portion, so that the whole form of the ball is such
as would be produced by cutting off a segment from the
front of a spheroid of the diameter of the sclerotic, and
replacing this by a segment cut from a smaller, and con-
sequently more convex, spheroid.

^54

ELEMENTARY PHYSIOLOGY.

[less.

15. The corneo-sclerotic case of the eye is kept in shape
by what are termed the /iu//wurs—\va.tery or semi-fluid
substances, one of which, the aqueous humour (Fig. 76, 7'),

Fig. 76. — Horizontal Section of the Eyeball.
I, cornea ; i', conjunctiva ; 2, sclerotic ; 2', sheath of optic nerve; 3, choroid;
3"; rods and cones of the retina ; 4, ciliarj' muscle ; 4', circular portion of
ciliary muscle ; 5, ciliarj- process ; 6, posterior chamber between ; 7, the
iris and the suspensory' ligament ; 7', anterior chamber ; 8, artery of retina
in the centre of the optic nerve ; 8', centre of blind spot ; 8", macula lutea ;
p, ora serrata (this is of course not seen in a section such as this, but is
introduced to show its position) ; 10, space behind the suspensory ligament
(canal of Petit) ; 12, crystalline lens ; 13, vitreous humour ; 14, marks the
position of the ciliarj- ligament ; a, optic axis, (in the actual eye of which
this is an exact copy, the yellow spot happened, curiously enough, not to be
in the optic axis) ; b, line of equator of the eyeball.

which is hardly more than water holding a few organic
and saline substances in solution, distends the cor-
neal chamber of the eye, while the other, the vitreous

IX.] THE CHOROID COAT. 255

(Fig. 76, 13), which is rather a deHcate jelly than a regular
fluid, keeps the sclerotic chamber full.

The two humours are separated by the very beautiful,
transparent, doubly-convex crystaUi7ie le?is (Fig. 76, 12),
denser, and capable of refracting light more strongly than
either of the humours. The cr}'stalline lens is composed
of fibres having a somewhat complex arrangement, and is
highly elastic. It is more convex behind than in front,
and it is kept in place by a delicate, but at the same time
strong membranous frame or suspensory ligament^ which
extends from the edges of the lens to what are termed
the ciliary processes of the choroid coat (Figs. 76, 5, and
77, c\ In the ordinary condition of the eye this ligament
is kept tense, i.e. is stretched pretty tight, and the front
part of the lens is consequently flattened.

16. This cho7'oid coat (Fig. 76, 3) is a highly vascular
membrane, in close contact with the sclerotic externally,
and lined, internally, by a layer of small polygonal bodies
containing much pigmentary matter, called pigDient cells
(Fig. 75). These pigment cells are separated from the
vitreous humour by the retina only. The rods and cones
of the latter are in immediate contact with them ; indeed
these cells may perhaps, be more truly considered as part
of the retina than as part of the choroid. The choroid
lines ever>- part of the sclerotic, except just where the
optic nerve enters it at a point below, and to the inner
side of the centre of the back of the eye ; but when it
reaches the front part of the sclerotic, its inner surface
becomes raised up into a number of longitudinal ridges,
with intervening depressions, like the crimped frills of a
lady's dress, terminating within and in front by rounded
ends, but passing, externally, into the iris. These ridges,
which when viewed from behind seem to radiate on all
sides from the lens (Figs. ']']^ c, and 76, 5), are the above-
mentioned ciliar}- processes.

17. The iris itself (Figs. 76, ~, and yj, a, b) is, as has
been already said, a curtain with a round hole in the
middle, provided with circular and radiating unstriped
muscular fibres, and capable of having its central aperture
enlarged or diminished by the action of these fibres, the
contraction of which, unlike that of other unstriped mus-
cular fibres, is extremely rapid. The edges of the iris are

256

ELEMENTARY PHYSIOLOGY.

[less.

firmly connected with the capsule of the eye, at the junc-
tion of the cornea and sclerotic, by the connective tissue
which enters into the composition of the so-called ciliary
ligament. Unstriped muscular fibres, having the same
attachment in front, spread backwards on to the outer
surface of the choroid, constituting the ciliary muscle
(Fig. 76, 4). If these fibres contract, it is obvious that
they will pull the choroid forwards ; and as the frame, or
suspensory ligament of the lens, is connected with the
ciliary processes (which simply form the anterior termina-
tion of the choroid), this pulling forward of the choroid
comes to the same thing as a relaxation of the tension of

Fig. 77. — View of Front Half of the Eyeball seen from' behind.

a, circular fibres ; b, radiating fibres of the iris ; c, ciliary processes ;

</, choroid. The cr>-stalline lens has been removed.

that suspensory ligament, which, as I have just said, is in
an ordinar)^ condition stretched somewhat tight, keeping
the front of the lens flattened.

The iris does not hang down perpendicularly into the
space between the front face of the crystalline lens and
the posterior surface of the cornea, which is filled by
the aqueous humour, but applies itself very closely to the
anterior face of the lens, so that hardly any interval is left
between the two (Figs. 76 and 78).

The retina, as we have seen, lines the interior of the eye,
being placed between the choroid and vitreous humour,

IX.] ADJUSTMENT. 257

its rods and cones being imbedded in the pigment epithe-
lium lining the former, and its inner limiting membrane
touching the latter.

About a third of the distance back from the front of the
eye the retina seems to end in a wavy border called the
ora serrata (Fig. 76, 9), and in reality the nervous ele-
ments of the retina do end here, having become consider-
ably reduced before this line is reached. Some of the
connective tissue elements however pass on as a delicate
kind of membrane at the back of the ciliar)- processes
towards the cr)-stalline lens.

18. The eyeball, the most important constituents of
which have now been described, is, in principle, a camera
of the kind described above— a water camera. That is to
say, the sclerotic answers to the box, the cornea to the
watch-glass, the aqueous and vitreous humours to the
water tilling the box, the cr)-stalline to the glass lens, the
introduction of which was imagined. The back of the
box corresponds with the retina.

But further, in an ordinan.' camera obscura, it is found
desirable to have what is termed a diaphragm (that is, an
opaque plate with a hole in its centre) in the path of the
rays, for the purpose of moderating the light and cutting
off the marginal rays which, owing to certain optical pro-
perties of spheroidal surfaces, give rise to defects in the
image formed at the focus.

In the eye, the place of this diaphragm is taken by the
iris, which has the peculiar advantage of being self-regu-
lating : dilating its aperture, and admitting more hght
when the light is weak ; but contracting its aperture and
admitting less light when the illumination is strong.

19. In the water camera, constructed according to the
description given above, there is the defect that no provi-
sion exists for adjusting the focus to the varying distances
of objects. If the box were so made that its back, on
which the image is supposed to be thrown, received distinct
images of very distant objects, all near ones would be
indistinct. And if, on the other hand, it were fitted to
receive the image of near objects, at a given distance,
those of either still nearer, or more distant, bodies would
be blurred and indistinct. In the ordinary- camera this

2SS ELEMENTARY PHYSIOLOGY. [less.

difficulty is overcome by sliding the lenses in and out, a
process which is not compatible with the construction of
our water camera. But there is clearly one way among
many, in which this adjustment might be effected — namely,
by changing the glass lens ; putting in a less convex one
when more distant objects had to be pictured, and a more
convex one when the images of nearer objects were to be
thrown upon the back of the box.

But it would come to the same thing, and be much
more convenient, if, without changing the lens, one and
the same lens could be made to alter its convexity. This
is what actually is done in the adjustment of the eye to
distances.

20. The simplest way of experimenting on the adjust-
ine7it or accommodation of the eye is to stick two stout
needles upright into a straight piece of wood, not exactly,
but nearly in the same straight line, so that, on applying
the eye to one end of the piece of wood, one needle ia)
shall be seen about six inches off, and the other ib) just on
one side of it at twelve inches or more distance.

If the observer look at the needle b, he will find that
he sees it very distinctly, and without the least sense of
effort ; but the image of a is blurred and more or less
double. Now let him tr}- to make this blurred image of
the needle a distinct. He will find he can do so readily
enough, but that the act is accompanied by a sense of
effort somewhere in the eye. And in proportion as a
becomes distinct, b will become blurred. Nor will any
effort enable him to see a and b distinctly at the same
time.

21. Multitudes of explanations have been given of this
remarkable power of adjustment ; but the true solution of
the problem has been gained by the accurate determina-
tion of the nature of the changes in the eye which
accompany the act. When the flame of a taper is held
near, and a little on one side of, a person's eye, any one
looking into the eye from a proper point of view, will
see three images of the flame, two upright and one in-
verted. One upright figure is reflected from the front
of the cornea, which acts as a convex mirror. The
second proceeds from the front of the crystalline lens,
which has the same effect ; while the inverted image

IX.]

ADJUSTMENT.

259

proceeds from the posterior face of the lens, which, being
convex backwards, is, of course, concave forwards, and
acts as a concave mirror.

Suppose the eye to be steadily fixed on a distant object,
and then adjusted to a near one in the same line of vision,
the position of the eyeball remaining unchanged. Then
the upright image reflected from the surface of the cornea,
and the inverted image from the back of the lens, will
remain unchanged, though it is demonstrable that their
size or apparent position must change if either the cor-
nea, or the back of the lens, alter either their form or their
position. But the second upright image, that reflected by
the front face of the lens, does change both its size and its
position ; it comes forward and grows smaller, proving
that the front face of the lens has become more convex.

Fig. 78.

Illustrates the change in the form of the lens when adjusted — A to distant,
B to near objects.

The change of form of the lens is, in fact, that represented
in Fig. 78.

These may be regarded as the facts of adjustment with
which all explanations of that process must accord. They
at once exclude the hypothesis (i) that adjustment is the
result of the compression of the ball of the eye by its
muscles, which would cause a change in the form of the
cornea ; (2) that adjustment results from a shifting of the
lens bodily, for its hinder face does not move ; (3) that it
results from the pressure of the iris upon the front face of
the lens, for under these circumstances the hinder face of
the lens would not remain stationary. This last hypo-
thesis is further negatived by the fact that adjustment takes
place equally well when the iris is absent.

s 2

26o ELEMENTARY PHYSIOLOGY. [less.

One other explanation remains, which is, not only ex-
ceedingly probable from the anatomical relations of the
parts, but is also supported by direct experimental evi-
dence. The lens, which is very elastic, is kept habitually
in a state of tension by the pressure exerted by its sus-
pensory ligament, and consequently has a flatter form
than it would take if left to itself. If the ciliary muscle
contracts, it must, as has been seen, relax that ligament,
and thereby diminish its pressure upon the lens. The
lens, consequently, will become more convex ; it will,
however, return to its former shape when the ciliar)'-
muscle ceases to contract, and allows the choroid to
return to its ordinary place.

Hence probably the sense of effort we feel when we
adjust for near distances arises from the contraction of
the ciliary muscle.

' 22. Adjustment can take place only within a certain
range ; this, however, admits of great individual varia-
tions.

People possessing ordinary or as it is called " normal "
sight can adjust their eyes so as to see distinctly objects as
near to the eye as five or six inches ; but the image of an
object brought nearer than this becomes blurred and indis-
tinct, because the "near limit'' of adjustment is then
passed. They can also adjust their eyes for objects at a
very great distance, the indistinctness of the images of
objects very far off being due not to want of proper focus-
sing, but to the details being lost through the minuteness
of the image.

Some people, however, are born with, or at least come
to possess eyes, in which the "near limit" of adjust-
ment is much closer. Such persons can see distinctly ob-
jects as near to the cornea as even one or two inches ;
but they cannot adjust their eyes to objects at any great
distance off. Thus many of these "near-sighted" people,
as they are called, cannot see distinctly the features of a
person only a few feet off. Though their ciliary muscle
remains quite relaxed so that the suspensory ligament
keeps the lens as flat as possible, the arrangements of the
eye are such that the image of an object only a few feet
off is brought to a focus before the retina, somewhere in
the vitreous humour. By wearing concave glasses these

ixO MUSCLES OF THE EVE. 26i

near-sighted people are able to bring the image of distant
objects on to the retina and thus to see them distinctly.

The cause of near-sightedness is not always the same,
but in the majority of cases it appears to be due to the
bulb of the eye being unusually long from back to front.
If, in the water camera described above, when the lens and
object were so adjusted that the image of the object was
distinctly focussed on the screen, the box were made
longer, so that the screen was moved backwards, the
distinctness of the image on it would be lost.

Some people are bom really " long-sighted.'' inasmuch
as they can see distinctly only such objects as are quite
distant ; and indeed have to contract their ciliarv- muscles,
and so make their lens more convex even to see these. Near
objects they cannot see distinctly at all unless they use
convex glasses. In such persons the bulb of the eye is
generally too short.

A kind of long-sightedness also comes on in old people ;
but this is different from the above, and is simply due, in
the majority of cases at all events, to a loss of power of
adjustment. The refractive power of the eye remains the
same, but the cilian.- muscle fails to work : and hence ad-
justment for near objects becomes impossible, though
distant objects are seen as before. For near objects such
persons have to use convex glasses. They should perhaps
be called " old-sighted " rather than ''' long-sighted."

In the water camera the image brought to a focus on
the screen at the back is ifii'erted ; the image of a tree for
instance is seen with the roots upwards and the leaves and
branches hanging downwards. The right of the image
also corresponds with the left of the object and vice versa.
Exactly the same thing takes place in the eye with the
ima^e focussed on the retina. It too is inverted. CSee
Lesson X. § ii.)

23. The jnusdes which move the eyeball are altogether
six in number — four straight muscles, or recti, and two
obhque muscles, the obliqui (Fig. 79). The straight
muscles are attached to the back of the bony orbit, round
the edges of the hole through which the optic ner\e
passes, and run straight forward to their insertions into
the sclerotic — one, the superior rectus, in the middle line
above ; one, the inferior, opposite it below ; and one

262

ELEMENTARY PHYSIOLOGY.

[less.

half-way on each side, the external and internal recti. The
eyeball is completely imbedded in fat behind and later-
ally ; and these muscles turn it as on a cushion ; the su-
perior rectus inclining the axis of the eye upwards, the
inferior downwards, the external outwards, the internal
inwards.

The two oblique muscles, upper and lower, are both
attached on the outer side of the ball, and rather behind
its centre ; and they both pull in a direction from the
point of attachment towards the inner side of the orbit —
the lower, because it arises here ; the upper, because,

Fig. 79.
A, the muscles of the right eyeball viewed from above, and B of the left
eyeball viewed from the outer side; S.R. the superior xe.c\.\s%\ Inf.R. the
inferior rectus ; E.R., In.R. the external rectus ; S.Ob, the superior oblique ;
Inf. Ob. the inferior oblique ; Ch. the chiasma of the optic ner\es (//.); ///,
the third ner\'e which supplies all the muscles except the superior oblique and
the e.\ternal rectus.

though it arises along with the recti from the back of the
orbit, yet, after passing forwards and becoming tendinous
at the upper and inner corner of the orbit, it traverses a
pulley-like loop of ligament, and then turns downwards
and outwards to its insertion. The action of the oblique
muscles is somewhat complicated, but their general ten-
dency is to roll the eyeball on its axis, and pull it a little
forward and inward.

24. The eyelids are folds of skin containing thin plates
of ca.tilage, and fringed at the edges with hairs, the eye-

ix.i

THE LACHRYMAL APPARATUS.

253

lashes^ and with a series of small glands called Meibomian.
Circularly disposed fibres of striped muscle lie beneath
the integuments of the eyelids, and constitute the orbi-
cularis muscle which shuts them. The upper eyelid is
raised by a special muscle, the levator of the upper lid
which arises at the back of the orbit and runs forwards to
end in the lid.

The lower lid has no special depressor.

25. At the edge of the eyelids the integument becomes
continuous with a delicate, vascular and highly nervous
mucous membrane, the conjunctiva, which lines the in-
terior of the lids and the front of the eyeball, its epithelial

ysr.os.

M-or3.

^Tn/tOO.

Fig. 80.
The front view of the right eye dissected to show, Orb., the orbicular
muscle of the eyelids ; the pulley and insertion of the superior oblique, S.Ob.,
and the inferior oblique, Iti/.Oh. ; L.G., the lachrymal gland.

layer being even continued over the cornea. The nume*
rous small ducts of a gland which is lodged in the orbit,
on the outer side of the ball (Fig. 80, L.G.), the lachrymal
gland, constantly pour its watery secretion into the inter-
space between the conjunctiva lining the upper eyelid and
that covering the ball. On the inner side of the eye is a
reddish fold, the caruncula lachrymalis, a sort of rudi-
ment of that third eyelid which is to be found in many
animals. Above and below, close to the caruncular, the
edge of each eyelid presents a minute aperture (the
pufictum lachryjnale), the opening of a small canal. The

264

ELEMENTARY PHYSIOLOGY.

[less.

canals from above and below converge and open into the
lachrymal sac ; the upper blind end of a duct {L.D., Fig.
81) which passes down from the orbic to the nose, open-
ing below the inferior turbinal bone (Fig. 40, h). It is
through this system of canals that the conjunctival mucous
membrane is continuous wath that of the nose ; and it is

-X.G.

Fig. 81.

A front view of the left eye, with the eyelids partially dissected to show

lachrymal gland, L.G., and lachrymal duct, L.D.

by them that the secretion of the lachrymal gland is ordin-
arily carried away as fast as it forms.

But, under certain circumstances, as when the con-
junctiva is irritated by pungent vapours, or when painful
emotions arise in the mind, the secretion of the lachrymal
gland exceeds the drainage power of the lachrymal duct,
and the fluid, accumulating between the lids, at length
overflows in the form of tears.

X.] SIMPLE AND COMPOUND SENSATIONS. 265

LESSON X.

THE COALESCENCE OF SENSATIONS WITH ONE
ANOTHER AND WITH OTHER STATES OF CON-
SCIO US NESS.

1, In explaining the functions of the sensor>^ organs,
I have hitherto confined myself to describing the means
by which the physical agent of a sensation is enabled
to irritate a given sensory nerve ; and to giving some
account of the simple sensations which are thus
evolved.

Simple se7isatioiis of this kind are such as might be
produced by the irritation of a single ner^-e-fibre, or of
several nerve-fibres by the same agent. Such are the
sensations of contact of warmth, of sweetness, of an odour,
of a musical note, of whiteness, or redness.

But very few of our sensations are thus simple. Most
of even those which we are in the habit of regarding
as simple, are really compounds of different simultaneous
sensations, or of present sensations with past sensations,
or with those feelings of relation which form the basis of
judgments. For example, in the preceding cases it is very
difficult to separate the sensation of contact from the
judgment that something is touching us ; of sweetness,
from the idea of something in the mouth ; of sound or
light, from the judgment that something outside us is
shining, or sounding.

2. The sensations of smell are those which are least
complicated by accessories of this sort. Thus, particles
of musk diffuse themselves with great rapidity through
the nasal passages, and give rise to the sensation of

266 ELKMP:NTARY PHYStOLOGY. [less.

a powerful odour. But beyond a broad notion that
the odour is in the nose, this sensation is unaccompanied
by any ideas of locaHty and direction. Still less does
it give rise to any conception of form, or size, or force,
or of succession, or contemporaneity. If a man had
no other sense than that of smell, and musk were the
only odorous body, he could have no sense of outness —
no power of distinguishing between the external world and
himself.

3. Contrast this with what may seem to be the equally
simple sensation obtained by drawing the finger along
the table, the eyes being shut. This act gives one the
sensation of a flat, hard surface outside oneself, which
sensation appears to be just as simple as the odour of
musk, but is really a complex state of feeling compounded
of—

ia) Pure sensations of contact.

{b) Pure muscular sensations of two kinds, — the one
arising from the resistance of the table, the other from the
actions of those muscles which draw the finger along,

{c) Ideas of the order in which these pure sensations
succeed one another.

id) Comparisons of these sensations and their order,
with the recollection of like sensations similarly arranged,
which have been obtained on previous occasions.

{e) Recollections of the impressions of extension, flat-
ness, &c. made on the organ of vision when these previous
tactile and muscular sensations were obtained.

Thus, in this case, the only pure sensations are those
of contact and muscular action. The greater part of what
we call the sensation is a complex mass of present and
recollected sensations and judgments.

4. Should any doubt remain that we do thus mix up
our sensations with our judgments into one indistinguish-
able whole, shut the eyes as before, and, instead of
touching the table with the finger, take a round lead
pencil between the fingers, and draw that along the table.
The " sensation " of a flat hard surface will be just as
clear as before ; and yet all that we touch is the round
surface of the pencil, and the only pure sensations we
owe to the table are those afforded by the muscular sense.
In fact, in this case, our " sensation " of a flat hard

X.] JUDGMENTS AND SENSATIONS. 267

surface is entirely a judgment based upon what the
muscular sense tells us is going on in certain muscles.

A still more striking case of the tenacity with which
we adhere to complex judgments, which we conceive to
be pure sensations, and are unable to analyse otherwise
than by a process of abstract reasoning, is afforded by our
sense of roundness.

Any one taking a marble between two fingers will say
that he feels it to be a single round body ; and he will
probably be as much at a loss to answer the question how
he knows that it is round, as he would be if he were asked
how he knows that a scent is a scent.

Nevertheless, this notion of the roundness of the
marble is really a very complex judgment, and that it is
so may be shown by a simple experiment. If the index
and middle fingers be crossed, and the marble placed
between them, so as to be in contact with both, it is
utterly impossible to avoid the belief that there are two
marbles instead of one. Even looking at the marble,
and seeing that there is only one, does not weaken
the apparent proof derived from touch that there are
two.^

The fact is, that our notions of singleness and round-
ness are, really, highly complex judgments based upon
a few simple sensations ; and when the ordinary conditions
of those judgments are reversed, the judgment is also
reversed. 1

With the index and the middle fingers in their ordinary
position, it is of course impossible that the outer sides
of each should touch opposite surfaces of one spheroidal
body. If, in the natural and usual position of the fingers,
their outer surfaces simultaneously give us the impression
of a spheroid (which itself is a complex judgment), it is
in the nature of things that there must be two spheroids.
But, when the fingers are crossed over the marble, the
outer side of each finger is really in contact with a
spheroid ; and the mind, taking no cognizance of the
crossing, judges in accordance with its universal

^ A ludicrous form of this experiment is to apply the crossed fingers to the
end of the nose, when it at once appears double ; and in spite of the
absurdity of the conviction, the mind cannot expel it, so long as the seQsa«
tions last.

268 ELEMENTARY PHYSIOLOGY. [less.

experience, that two spheroids, and not one, give rise to
the sensations which are perceived.

5. Phenomena of this kind are not uncommonly called
delusions of the senses j but there is no such thing as a
fictitious, or delusive, sensation, A sensation must exist
to be a sensation, and, if it exists, it is real and not de-
lusive. But the judgments we form respecting the causes
and conditions of the sensations of which we are aware,
are very often erroneous and delusive enough ; and such
judgments may be brought about in the domain of every
sense, either by artificial combinations of sensations,
or by the influence of unusual conditions of the body
itself. The latter give rise to what are called subjective
sensations.

JVIankind would be subject to fewer delusions than they
are, if they constantly bore in mind their liability to false
judgments due to unusual combinations, either artificial
or natural, of true sensations. Alen say, " I felt,'*' " I
heard,'' " I saw " such and such a thing, when, in ninety-
nine cases out of a hundred, what they really mean is,
that they judge that certain sensations of touch, hearing,
or sight, of which they were conscious, were caused by
such and such things.

6. Among subjective sensations within the domain of
touch, are the feelings of creeping and prickling of the
skin, which may sometimes be due to certain states of the
circulation, but probably, more frequently to processes
going on in the central nervous system. The subjective
evil smells and bad tastes which accompany some diseases
are, in a similar way, very probably due to disturbances in
the brain in the central end-organs of the nerves of smell
and ta«te.

Many persons are liable to what may be called auditory
spectra — music of various degrees of complexity sounding
in their ears, without any external cause, while they are
wide awake. I know not if other persons are similarly
troubled, but in reading books written by persons with
whom I am acquainted, I am sometimes tormented by
hearing the words pronounced in the exact way in which
these persons would utter them, any trick or peculiarity
of voice, or gesture, being, also, very accurately repro-
duced. And I suppose that everj^ one must have

X.] OCULAR SPECTRA. 269

been startled, at times, by the extreme distinctness
with which his thoughts have embodied themselves in
apparent voices.

The most wonderful exemplifications of subjective sen-
sation, however, are afforded by the organ of sight.

Any one who has witnessed the sufferings of a man
labouring under delirium trenie?ts (a disease produced by
excessive drinking), from the marvellous distinctness of
his visions, which sometimes take the forms of devils,
sometimes of creeping animals, but almost always of
something fearful or loathsome, will -not doubt the inten-
sity of subjective sensations in the domain of vision.

7. But in order that illusive visions of great distinctness
should appear, it is not necessary for the nervous system
to be thus obviously deranged. People in the full
possession of their faculties, and of high intelligence, may
be subject to such appearances, for which no distinct'
cause can be assigned. An excellent illustratioli of this
is the famous case of Mrs. A, given by Sir David Brewster,
in his Natural Magic. This lady was subject to un-,
usually vivid auditory and ocular spectra. Thus on one,
occasion she saw her husband standing before her and
looking fixedly at her with a serious expression, though
at the time he was at another place. On another occa-
sion she heard him repeatedly call her, though at the
time he was not anywhere near. On another occasion
she saw a cat in the room lying on the rug ; and so vivid^
was the illusion that she had great difficulty in satisfying,
herself that really there was no cat there. The whole
account is well worthy of perusal.

It is obvious that nothing but the singular courage and
clear intellect of Mrs. A. prevented her from becoming a
mine of ghost stories of the most excellently authenticated
kind. And the particular value of her history lies in
its showing, that the clearest testimony of the most
unimpeachable witness may be quite inconclusive as to
the objective reality of something which the witness has
seen.

Mrs. A. undoubtedly saw what she said she saw. The
evidence of her eyes as to the existence of the apparitions,
and of her ears to those of the voices, was, in itself, as
perfectly trustworthy as their evidence would have been

270 ELEMENTARY PHYSIOLOGY. [less.

had the objects really existed. For there can be no doubt
that exactly those parts of her retina which would have
been affected by the image of a cat, and those parts of
her auditory organ which would have been set vibrating
by her husband's voice, or the portions of the sensorium
with which those organs of sense are connected, were
thrown into a corresponding state of activity by some
internal cause.

What the senses testify is neither more nor less than the
fact of their own affection. As to the cause of that affec-
tion they really say nothing, but leave the mind to form
its own judgment on the matter. A hasty or superstitious
person in Mrs. A.'s place would have formed a wrong
judgment, and would have stood by it on the plea that
" she must believe her senses."

8. The delusions of the judgment, produced not by ab-
normal conditions of the body, but by unusual or artificia/
combinations of sensations, or by suggestions of ideas, are
exceedingly numerous, and, occasionally are not a little
remarkable.

Some of those which arise out of the sensation of touch
have already been noted. I do not know of any produced
through smell or taste, but hearing is a fertile source of
such errors.

What is called ve?itriloguism (speaking from the belly),
and is not uncommonly ascribed to a mysterious power
of producing voice somewhere else than in the larynx,
depends entirely upon the accuracy with which the per-
former can simulate sounds of a particular character, and
upon the skill with which he can suggest a belief in the
existence of the causes of these sounds. Thus, if the
ventriloquist desire to create the belief that a voice issues
from the bowels of the earth, he imitates wath great accu-
racy the tones of such a half-stifled voice, and suggests
the existence of some one uttering it by directing his
answers and gestures towards the ground. These gestures
and tones are such as would be produced by a given
cause ; and no other cause being apparent, the mind
of the bystander insensibly judges the suggested cause to
exist.

9. The delusions of the judgment through the sense of
sight — optical delusions^ as they are called — are more

X.J DELUSIONS OF THE JUDGMENT. 271

numerous than any others, because such a great number
of what we think to be simple visual sensations are really
very complex aggregates of visual sensations, tactile sen-
sations, judgments, and recollections of former sensations
and judgments.

It will be instructive to analyse some of these judgments
into their principles, and to explain the delusions by the
application of these principles.

10, When we look at an external object^ the image of
the object falls on the retina at the end of the visual
axis, i.e., a lifie joining the object and the retina and
traversing a particular region of the centre of the eye.
Conversely, luhen a part of the retina is excited, by what-
ever means, the sensatioji is referred by the mind to some
cause outside the body in the directioji of the visual axis.

Whe?t we look at an external object which is felt by
the touch to be in a given place, the linage of the object
falls upo7i a certain part of the retina. Conversely, when
a part of the retina is excited, by whatever meaiis, the
sensatio7i is referred by the mi)id to some cause outside the
body occupy i fig such a position that its linage would fall
on that part.

It is for this reason that when a phosphene is created
by pressure, say on the outer and lower side of the eye-
ball, the luminous image appears to lie above, and to the
inner side of, the eye. Any external object which could
produce the sense of light in the part of the retina pressed
upon must, owing to the inversion of the retinal images
(see Lesson IX. § 23), in fact occupy this position ; and
hence the mind refers the light seen to an object in that
position.

11. The same kind of explanation is applicable to the
apparent paradox that, while all the pictures of external
objects are certainly inverted on the retina by the refract-
ing media of the eye, we nevertheless see them upright.
It is difficult to understand this, until one reflects that the
retina has, in itself, no means of indicating to the mind
which of its parts lies at the top, and which at the bottom ;
and that the mind learns to call an impression on the
retina high or low, right or left, simply on account of the
association of such an impression with certain coincident
tactile impressions. In other words, when one part of the

272 ELEMENTARY PHYSIOLOGY. [less.

retina is affected, the object causing the affection is found
to be near the right hand ; when another, the left ; when
another, the hand has to be raised to reach the object ;
when yet another, it has to be depressed to reach it. And
thus the several impressions on the retina are called
right, left, upper, lower, quite irrespectively of their real
positions, of which the mind has, and can have, no
cognizance.

12. When a?i external body is ascertai7iedby touch to be
simple^ it forms but o?ie image on the reti?ia of a single
eye j afid when two or more images fall on the reti?ia of
a single eye, they ordinarily proceed f'om a corresponding
number of bodies which are distinct to the touch,

Cofiversely, the sensation of two or more images is
judged by the mind to proceed from two or more objects.

If two pin-holes be made in a piece of cardboard at a
distance less than the diameter of the pupil, and a small
object like the head of a pin be held pretty close to the
eye, and viewed through these holes, two images of the
head of the pin will be seen. The reason of this is, that
the rays of light from the head of the pin are split by the
card into two minute pencils, which pass into the eye on
either side of its centre, and, on account of the nearness of
the pin to the eye, meet the retina before they can be
imited again and brought to one focus. Hence they fall
on different parts of the retina, and each pencil of rays
being very small, makes a tolerably distinct image of its
own of the pin's head on the retina. Each of these images
is now referred outward (§ lo) and two pins are apparently-
seen instead of one. A like explanation applies to multi-
plying glasses and doubly refracti?ig crystals, both of
which, in their own ways, split the pencils of light pro-
ceeding from a single object into two or more separate
bundles. These give rise to as many images, each
of which is referred by the mind to a distinct external
object.

13. Certain visual phe7iomena ordinarily accompany
those products of tactile sensation to which we give the
name of size, distance, and form. Thus, other things
being alike, the space of the retina covered by the image of
a large object is larger than that covered by a small object ;
whiU that covered by an object when near is larger

X.] DELUSIONS OF THE JUDGMENT. 273

than that covered by the same object when distant ; and,
other conditions being alike, a near object is jnore brilliant
than a distant one. Furthermore, the shadows of objects
differ according to the forms of their surfaces, as determined
by touch.

Conversely , if these visual sensations can be produced,
they inez'itably suggest a belief in the existence of objects
competent to produce the corresponding tactile sensatiofis.

What is called perspective, whether solid or aerial in
drawing, or painting, depends on the application of these
principles. It is a kind of visual ventriloquism — the
painter putting upon his canvas all the conditions requisite
for the production of images on the retina, having the size,
relative form, and intensity of colour of those which would
actually be produced by the objects themselves in nature.
And the success of his picture, as an imitation, depends
upon the closeness of the resemblance between the images
it produces on the retina, and those which would be pro-
duced by the objects represented.

14. To most persons the image of a pin, at three or
four inches from the eye, appears blurred and indistinct
— the eye not being capable of adjustment to so short a
focus. If a small hole be made in a piece of card, the
circumferential rays which cause the blur are cut off, and
the image becomes distinct. But at the same time it is
magnified, or looks bigger, because the image of the pin,
in spite of the loss of the circumferential rays, occupies a
much larger extent of the retina when close than when
distant. All convex glasses produce the same effect —
while concave lenses diminish the apparent size of an
object, because they diminish the size of its image on the
retina.

15. The moon, or the sun, when near ihe horizon ap-
pears ver}' much larger than when it is high in the sky.
When in the latter position, in fact, we have nothing
to compare it with, and the small extent of the retina
which itS- image occupies suggests small absolute size.
But as it sets, we see it passing behind great trees
and buildings which we know to be very large and
very distant, and yet it occupies a larger space on the
retina than they do. Hence the vague suggestion of
its larger size.

274 ELEMENTARY PHYSIOLOGY. [less.

16. If a convex surface be lighted from one side, the side
towards the hght is bright— that turned from the hght,
dark, or in shadow ; while a concavity is shaded on the
side towards the light, bright on the opposite side.

If a new half-crown, or a medal with a well-raised head
upon its face, be lighted sideways by a candle, we at once
know the head to be raised (or a cameo) by the disposition
of the light and shade ; and if an intaglio, or medal on
which the head is hollowed out, be lighted in the same
way, its nature is as readily judged by the eye.

But now, if either of the objects thus lighted be viewed
with a convex lens, which inverts its position, the light and
dark sides will be rev^ersed. With the reversal the judg-
ment of the mind will change, so that the cameo will be
regarded as an intaglio, and the intaglio as a cameo ;
for the light still comes from where it did, but the cameo
appears to have the shadows of an intaglio, and vice versa.
So completely, however, is this interpretation of the facts
a matter of judgment, that if a pin be stuck beside the
medal so as to throw a shadow, the pin and its shadow,
being reversed by the lens, will suggest that the direction
of the light is also reversed, and the medals will seem to
be wdiat they really are.

17. Whenever an external object is watched rapidly
changing its forni, a continuous series of different
pictures of the object is impressed upon the same spot of
the retina.

Conversely, if a continuous series of different pictures of
07ie object is impressed upon otie part of the retina, the
mind judges that they are due to a single external object,
undergoing changes of form.

This is the principle of the curious toy called the thaii-
inatrope, or " zootrope," or " wheel of life," by the help of
which, on looking through a hole, one sees images of
jugglers throwing up and catching balls, or boys playing
at leapfrog over one another's backs. This is managed
by painting at intervals, on a disk of card, figures and
jugglers in the attitudes of throwing, waiting to catch, and
catching ; or boys " giving a back,'' leaping, and coming
into position after leaping. The disk is then made to
rotate before an opening, so that each image shall be pre-
sented for an instant, and follow its predecessor before the

X.] SINGLE VISION WITH TWO EYES. 275

impression of the latter has died away. The result is that
the succession of different pictures irresistibly suggests one
or more objects undergoing successive changes — the juggler
seems to throw the balls, and the boys appear to jump over
one another's backs.

18. IVhen an external object is ascertained by touch to
be single, the centres of its retinal images in the two eyes
fall upon the centres of the y el low spots of the two eyes,
when both eyes are directed towards it ; but if there be two
external objects, the centres of both their images ca?inot
fall, at the same time, upon the centres of the yellow spots.

Conversely, when the centres of two images, formed
sifnultaneously in the two eyes, fall upon the centres of the
yellow spots, the mind judges the images to be caused by a
single external object ; but if not, by two.

This seems to be the only admissible explanation of the
facts, that an object which appears single to the touch and
when viewed with one eye, also appears single when it is
viewed with both eyes, though two images of it are neces-
sarily formed ; and on the other hand, that when the
centres of the two images of one object do not fall on the
centres of the yellow spots, both images are seen sepa-
rately, and we have double vision. In squinting, the axes
of the two eyes do not converge equally towards the object
viewed. In consequence of this, when the centre of the
image formed by one eye falls on the centre of the yellow
spot, the corresponding part of that formed by the other
eye does not, and double vision is the result.

For simplicity's sake we have supposed the images to
fall on the centre of the yellow spot But though vision is
distinct only in the yellow spot, it is not absolutely limited
to it ; and it is quite possible for an object to be seen as a
single object wiih two eyes, though its images fall on the
two retinas outside the yellow spots. All that is neces-
sary is that the two spots of the retinas on which the images
fall sliould be similarly disposed towards the centres of
their respective yellow spots. Any two points of the two
retinas thus similarly disposed towards their respective
yellow spots (or more exactly to the points in which the
visual axes end), are spoken of as corresponding points;
and any two images covering two corresponding areas are
conceived of as coming from a single object. It is obvious

T 2

276 ELEMENTARY PHYSIOLOCrV. [less.

that the inner (or nasal) side of one retina corresponds to
the outer (or cheek) side of the other.

19. In single vision ivith two eyes, the axes of the two
eyes, of the movements of which the 7nuscular sense gives
an indication, cut one another at a greater angle when the
object approaches, at a less angle when it goes further off.

Conversely, if without changing the position of an object,
the axes of the two eyes which view it can be made to con-
verge cr diverge, the object will seem to approach or go
further off.

In the instrument called the pseudoscope, mirrors or
prisms are disposed in such a manner that the angle at
which rays of light from an object enter the two eyes, can
be altered without any change in the object itself; and
consequently the axes of these eyes are made to converge or
diverge. In the former case the object seems to approach ;
in the latter, to recede.

20. When a body of moderate size, ascertained by touch
to be solid, is viewed with both eyes, the iinages of it,
formed by the two eyes, are necessarily different {one

shotuing more of its right side, the other of its left side).
Nevertheless, they coalesce into a co7nmon image, which
gives the impression of solidity.

Conversely, if the two images of the right and left
aspects of a solid body be inade to fall upon the retinas of
the two eyes in such a way as to coalesce into a common
linage, they are judged by the mind to proceed from the
single solid body which alone, under ordinary circian-
sta?ices, is competent to produce thein.

The stereoscope is constructed upon this principle.
Whatever its form, it is so contrived as to throw the
images of two pictures of a solid body, such as would be
obtained by the right and left eye of a spectator, on to
such parts of the retinas of the person who uses the
stereoscope as would receive these images, if they really
proceeded from one solid body. The mind immediately
judges them to arise from a single external solid body, and
sees such a solid body in place of the two pictures.

The operation of the mind upon the sensations presented
to it by the two eyes is exactly comparable to that which
takes place when, on holding a marble between the finger
and thumb, we at once declare it to be a single sphere

X.] JUDGMENT OF SOLIDITY. 277

(§ 4). That which is absolutely presented to the mind by
the sense of touch in this case is by no means the sensa-
tion of one spheroidal body, but two distinct sensations of
two convex surfaces. That these two distinct convexities
belong to one sphere, is an act of judgment, or process of
unconscious reasoning, based upon many particulars of
past and present experience, of which we have, at the
moment, no distinct consciousness.

278 ELEMENTARY PHYSIOLOGY. [less.

LESSON XL

THE NER VO US S VS TEM A ND INNER VA TION.

I. The sensory organs are, as we have seen, the chan-
nels through which particular physical agents are enabled
to excite the sensory nerves with which these organs are
connected ; and the activity of these nerves is evidenced
by that of the central organ of the nervous system, which
activity becomes manifest as a state of consciousness —
the sensation.

We have also seen that the muscles are instruments by
which a motor nerve, excited by the central organ with
which it is connected, is able to produce motion.

The sensory nerves, the motor nerves, and the central
organ, constitute the greater part of the 7iervoiis system^
which, with its function of i?inervation^ we must now study
somewhat more closely, and as a whole.

2. The nervous apparatus consists of two sets of nerves
and nerve-centres, which are intimately connected together
and yet may be conveniently studied apart. These are
the ce?'ebro-sptnal s)'stem and the sympathetic system.
The former consists of the ccrebro-spiiial axis (composed
of the brain and spiiial cord) and the cranial and spinal
Jterves, which are connected with this axis. The latter
comprises the chain of sympathetic ganglia, the nerves
which they give off, and the various cords by which they
are connected with one another and with the cerebro-
spinal nerves.

Nerves are made up entirely of nerve-fibres, the struc-
ture of which is somewhat different in the cerebro-spinal

XI.] THE SPINAL CORD. 279

and in the sympathetic systems. (See Lesson XI L)
Nerve centres, on the other hand, are composed of nerve-
cells mingled with nerve-tibres (Lesson XI L). Such
nerve-cells are found in various parts of the brain and
spinal cord, in the sympathetic ganglia, and also in the
ganglia belonging to spinal nerves as well as in certain
sensor>' organs, such as the retina and the internal ear.

3, The cerebrospinal axis lies in the cavity of the skull
and spinal column, the bony walls of which cavity are
lined by a very tough fibrous membrane, serving as the
periosteum of the component bones of this region, and
called the dura mater. The brain and spinal cord them-
selves are closely invested by a very vascular fibrous
tissue, called pia mater. The numerous blood vessels
supplying these organs run for some distance in the pia
mater, and where they pass into the substance of the
brain or cord, the fibrous tissue of the pia mater accom-
panies them to a greater or less depth.

Between the pia ?}iattr, and the dura inater^ lies
another delicate membrane, called the arachnoid mem-
brane. These three membranes are connected with each
other at various points, and the arachnoid, which is not
only very delicate, but also less regular than the other
two, divides the space between the dura and pia mater
into two spaces, each containing fluid, and each more or
less lined by a delicate epithelium. The space between
the dura mater and the arachnoid, often called the sub-
dural space, is nowhere ver}' large ; but the space between
the arachnoid and the pia mater, often called the sub-
arachnoid space, though small and insignificant in the
region of the brain, becomes large in the region of the
spinal cord, and here contains a considerable quantity of
fluid, called arachnoid or subarachnoid fluid.

4. The spinal cord (Fig. 82) -is a column of greyish-
white soft substance, extending from the top of the spinal
canal, where it is continuous with the brain, to about the
second lumbar vertebra, where it tapers off into a fila-
ment. A deep, somewhat broad, fissure, the anterior fissure
(Fig. 83, /), divides it in the middle line in front, nearly
down to its centre : and a similar deeper but narrower
cleft, the posterior fissure (Fig. 83, .^\ also extends nearly
to its centre in the middle line behind. The pia mater

2So

ELEMENTARY PHYSIOLOGY.

[less.

extends more or less into each of these fissures, and
supports the vessels which supply the cord with blood.
In consequence of the presence of these fissures, only a
narrow bridge of the substance of the cord connects its
two halves, and this bridge is traversed throughout its
entire length by a minute canal, the central canal of the
cord (Fig. 83, 3).

Each half of the cord is divided longitudinally into
three parts, the anterior, lateral, and posterior columns
(Fig. 83, 6, 7,8)^ by the lines of attachment of two parallel
series of delicate bundles of nervous filaments, the roots
of the spinal nerves. The roots of the ner\-es which arise

Fig. S2. — The Spinal Cord.

A. A front ^^ew of a portion of the cord. On the right side, the anterior
roots, A.R., are entire ; on the left side they are cut, to show the posterior
roots, P.R.

B. A transverse section of the cord. A, the anterior fissure ", P, the posterior
fissure ; G, the central canal ; C, the grey matter ; \V, the white matter ;
A.J?., the anterior root, P.R., the posterior root, Gn., the ganglion, and
T, the trunk, of a spinal nerve.

along that line which is nearer the posterior surface of the
cord are called posterior roots ; those which arise along
the other line are the anterior roots. A certain number
of anterior and posterior roots, on the same level on each
side of the cord, converge and form anterior and posterior
bundles, and then the two bundles, anterior and posterior,
coalesce into the tnmk of a spinal nerve ; but before doing
so, the posterior bundle presents an enlargement — tAe
ganglio7i of the posterior root.

The trunks of the spinal nerves pass out of the spinal
canal by apertures between the vertebrae, called the ////<v--
vertel/ral foraminaj :xnd. ih^n divide and subdivide, their

xr.]

THE SPINAL CORD.

281

ultimate ramifications going for the most part to the
muscles and to the skin.

There are thirty-one pair, of these spinal ncr\'es, and,

Fig. 83.— Tk.\nsver>e Section of one-half of the Spinal Cord (in
THE Lumbar Region), magnified.

I, anterior fissure; 2, posterior fissure ; 3, central canal ; 4 and 5, bridges
connecting the two halves (posterior and anterior commissures) ; 6, poste-
rior column ; 7, lateral column ; 8, anterior column ; 9, posterior root ; 10,
anterior root of nerve.

a a, posterior horn of grey matter ', e e e, anterior horn of grey matter.
Through the several columns 6, 7, and 8, each composed of white matter,
are seen the prolongations of the pia mater, which carrj- blood-vessels
into the cord Crom the outside. The pia mater itself is seen over the whole
of the cord.

282 ELEMENTARY PHYSIOLOGY. [less.

consequently, twice as many sets of roots of spinal nerves
given off, in two lateral series, from each half of the cord.

5. A transverse section of the cord (Fig. 82, B, and Fig.
83) shows that each half contains two substances— a white
substance on the outside, and a greyish-red substance in
the interior. And this grey matter, as it is called, is so
disposed that, in a transverse section, it looks, in each
half, something like a crescent, with one end bigger than
the other, and with the concave side turned outwards.
The two ends of the crescents are called its /ior?is or
cor?um (Fig. 83, e e), the one directed forwards being the
anterior cor 7iu ; the one turned backwards Xh^ posterior
cornii (Fig. 83, a a). The convex sides of the cornua of
the grey matter approach one another, and are joined by
the bridge which contains the central canal.

There is a fundamental difference in structure between
the grey and the white matter. The white matter consists
entirely of nerve-fibres supported in a delicate framework
of connective tissue, and accompanied by blood-vessels.
Most of these fibres run lengthways in the cord, and con-
sequently, in a transverse section, the white matter is
really composed of a multitude of the cut ends of these
fibres.

The grey matter, on the other hand, contains in addi-
tion a number of nerve-cells, some of them of considerable
size. These cells are wholly absent in the white matter.

Many of the nerve-fibres of which the anterior roots are
composed may be traced into the anterior cornu, and,
indeed, into the nerve-cells lying in the cornu, while those
of the posterior roots, for the most part, enter or pass
through the posterior cornu.

6. The physiological properties of the organs now
described are very remarkable.

If the tru7ik of a spinal nerve be irritated in any way, as
by pinching, cutting, galvanizing, or applying a hot body,
two things happen : in the first place, all the muscles to
which filaments of this nerve are distributed, contract ; in
the second, pain is felt, and the pain is referred to that
part of the skin to which fibres of the nerve are dis-
tributed. In other words, the effect of irritating the
trunk of a nerve is the same as that of irritating its
component fibres at their terminations.

XI.] FUNCTIONS OF NERVE ROOTS. 283

The effects just described will follow upon irritation of
any part of the brajiche^ of the nerve : except that when a
branch is irritated, the only muscles directly affected, and
the only region of the skin to which pain is referred, will
be those to which that branch sends nerve-fibres. And
these effects will follow upon irritation of any part of a
nerve from its smallest branches up to the point of its
trunk, at which the anterior and posterior bundles of root
fibres unite.

7. If the anterior bundle of root fibres be irritated in the
same way, only half the previous effects are brought
about. That is to say, all the muscles to which the nerve
is distributed contract, but no pain is felt.

So again if the posterior, ganglionated bundle be irri-
tated, only half the effects of irritating the whole trunk is
produced. But it is the other half ; that is to say, none
of the muscles to which the nerve is distributed contract,
but pain is referred to the whole area of skin to which the
fibres of the nerve are distributed.

8. It is clear enough, from these experiments, that all
the power of causing muscular contraction which a spinal
nerve possesses, is lodged in the fibres which compose its
anterior roots ; and all the power of giving rise to sensa-
tion, in those of its posterior roots. Hence the anterior
roots are commonly called motor, and the posterior
sensory.

The same truth may be illustrated in other ways. Thus,
if, in a living animal, the anterior roots of a spinal nerve
be cut, the animal loses all control over the muscles to
which that nerve is distributed, though the sensibility ot
the region of the skin supplied by the nerve is perfect.
If the posterior roots be cut, sensation is lost, and volun-
tary movement remains. But if both roots be cut, neither
voluntary movement nor sensibility is any longer possessed
by the part supplied by the nerve. The muscles are said
to be paralysed ; and the skin may be cut, or burnt, with-
out any sensation being excited.

If, when both roots are cut, that end of the motor root
which remains connected with the trunk of the nerve be
irritated, the muscles contract ; while, if the other end
be so treated, no apparent effect results. On the other
hand, if the end of the sensory root connected with the

284 ELEMENTARY PHYSIOLOGY. [less.

trunk of the nerve be irritated, no apparent effect is
produced, while, if the end connected with the cord be
irritated, pain immediately follows.

When no apparent effect follows upon the irritation of
any nerve, it is not probable that the molecules of the
nerve remain unchanged. On the contrary, it would
appear that the same change occurs in all cases ; but a
motor nerve is connected with nothing that can make that
change apparent save a muscle, and a sensory nerve with
nothing that can show an effect but the central nervous
system.

9. It will be observed that in all the experiments men-
tioned there is evidence that, when a nerve is irritated, a
something, probably, as we have seen (Lesson V., § 32), a
change in the arrangement of its molecules, is propagated
along the nerve-fibres. If a motor or a sensory nerve be
irritated at any point, contraction in the muscle, or sensa-
tion or (some other corresponding event) in the central
organ, immediately follows. But if the nerve be cut, or
even tightly tied at any point between the part irritated and
the muscle or central organ, the effect at once ceases, just
as cutting a telegraph wire stops the transmission of the
electric current or impulse. When a limb, as we say,
" goes to sleep," it is frequently because the nerves supply-
ing it have been subjected to pressure sufficient to destroy
the nervous ^ continuity of the fibres. We lose voluntary
control over, and sensation in, the limb, and these powers
are only gradually restored as that nervous continuity
returns.

Having arrived at this notion of an impulse travelling
along a nerve, we readily pass to the conception of a sensory
nerve as a nerve which, when active, brings an impulse to
the central organ, or is afferent; and of a motor nerve, as a
nerve which carries away an impulse from the organ, or
is efferent. It is very convenient to use these terms to

' Their " nen-ous continuity" — because their physical continuity is not
interrupted as a whole, but only that of the substance which acts as a con-
ductor of the nervous influence ; or, it may be that only the conducting
power of a part of that substance is interfered with. Imagine a telegraph
cable, made of delicate caoutchouc tubes, filled with mercury — a squeeze
would interrupt the "electrical continuity" of the cable, without destroying
its physical continuity. This analogy may not be exact, but it helps to make
the nervous phenomena intelligible.

XL] AFFERENT AND EFFERENT NERVES. 285

denote the two great classes ot nerves ; for, as vvc shall find
(§ 12), there are afferent nerves which are not sensory in
the sense of giving rise to a change of consciousness, or
sensation, while there are efferent nerves which are not
motor, in the sense of inducing muscular contraction.
The nerves, for example, by which the electrical fishes give
rise to discharges of electricity from peculiar organs to
which those nerves are distributed, are efferent, inasmuch
as they carry impulses to the electric organs, but are not
motor, inasmuch as they do not give rise to movements. The
pneumogastric when it stops the beat of the heart cannot be
called a motor, and yet is then acting as an efferent nerve.
It will, of course, be understood, as pointed out above, that
the use of these words does not imply that when a nerve
is irritated in the middle of its length, the impulses set up
by that irritation travel only away from the central organ
if the nerve be efferent, and towards it, if it be afferent.
On the contrary, we have evidence that in both cases the
impulses travel both ways. All that is meant is this, that
the afferent nerve from the disposition of its two ends, in
the skin, or other peripheral organs on the one hand,
and in the central organ on the other, is of use only
when impulses are travelling along it towards the central
organ, and similarly the efferent nerve is of use only when
impulses are travelling along it, away from the central organ.

10. There is no difference in structure, in chemical or
in physical character, between afferent and efferent nerves.
The impulse which travels along them requires a certain
time for its propagation, and is vastly slower than many
other movements — even slower than sound.

1 1. Up to this point our experiments have been confined
to the nerves. We may now test the properties of the
spinal cord in a similar way. If the cord be cut across
(say in the middle of the back), the legs and all the parts
supplied by nerves which come off below the section, will
be insensible, and no effort of the will can make them
move ; while all the parts above the section will retain
their ordinary powers.

When a man hurts his back by an accident, the cord is
not unfrequently so damaged as to be virtually cut in two,
and then paralysis and insensibility of the lower part of
the body ensue.

286 ELEMENTARY PHYSIOLOGY. [less.

If, when the cord is cut across in an animal, the cut
end of the portion below the division, or away from the
brain, be irritated, violent movements of all the muscles
supplied by nerves given off from the lower part of the
cord take place, but no sensation is felt by the brain. On
the other hand, if that part of the cord, which is still con-
nected with the brain, or better, if any afferent nerve
connected with that part of the cord be irritated, sensations
ensue, as is shown by the movements of the animal ; but
in these movements the muscles supplied by nerves com-
ing from the spinal cord below the cut take no part ; they
remain perfectly quiet.

12. Thus, it may be said that, in relation to the brain
the cord is a great mixed motor and sensory nerv^e. But
it is also much more.

For if the trunk of a spinal nerve be cut through, so as to
sever its connection with the cord, an irritation of the skin
to which the sensory fibres of that nerve are distributed,
produces neither motor nor sensory effect. But if the cord
be cut through anywhere so as to sever its connection with
the brain, irritation applied to the skin of the parts sup-
plied with sensory nerves from the part of the cord below
the section, though it gives rise to no sensation, may pro-
duce violent motion of the parts supplied with motor nerves
from the same part of the cord.

Thus, in the case supposed above, of a man whose legs
are paralysed and insensible from spinal injury, tickling
the soles of the feet will cause the legs to kick out convul-
sively. And as a broad fact, it may be said that, so long
as both roots of the spinal nerves remain connected w-ith
the cord, irritation of any afferent nerve is competent to
give rise to excitement of some, or the whole, of the efferent
ners'es so connected.

If the cord be cut across a second time at any'distaTice
below the first section, the efferent nerves below the" Second
cut will no longer be affected by irritation of the afferent
nerves above it — but only of those below the second
section. Or, in other words, in order that an afferent im-
pulse may be converted into an efferent one by the spinal
cord, the afferent nerve must be in uninterrupted material
communication with the efferent nerve, by means of the
substance of the spinal cord.

^].] REFLEX ACTIONS. 287

This peculiar power of the cord, by which it is com-
petent to convert afferent into efferent impulses, is that
which distinguishes it physiologically, as a central organ,
from a nerve, and is called rejiex action. It is a power
possessed by the grey matter, and not by the white
substance of the cord.

1 3. The number of the efferent nerves which may be ex-
cited by the reflex action of the cord, is not regulated alone
by the number of the afferent nerves which are stimulated
by the irritation which gives rise to the reflex action. Nor
does a simple excitation of the afferent nerve by any means
necessarily imply a corresponding simplicity in the ar-
rangement and succession of the reflected motor impulses.
Tickling the sole of the foot is a very simple excitation of
the afferent fibres of its nerves ; but in order to produce
the muscular actions by which the legs are drawn up, a
great multitude of efferent fibres must act in regulated
combination. In fact, in a multitude of cases, a reflex
action is to be regarded rather as the result of a dormant
activity of the spinal cord awakened by the arrival of the
afferent impulse, as a sort of orderly explosion fired off
by the afferent impulse, than as a mere rebound of
the afferent impulse into the first efferent channels open
to it.

The various characters of these reflex actions may be
very conveniently studied in the frog. If a frog be deca-
pitated, or, better still, if the spinal cord be divided close to
the head, and the brain be destroyed by passing a blunt
wire into the cavity of the skull, the animal is thus de-
prived (by an operation which, being almost instantaneous,
can give rise to very little pain) of all consciousness and
volition, and yet the spinal cord is left intact. At first the
animal is quite flaccid and apparently dead, no movement
of any part of the body (except the beating of the heart)
being visible. This condition, however, being the result
merely of the so-called shock of the operation, very soon
passes off, and then the following facts may be observed.

So long as the animal is untouched, so long as no
stimulus is brought to bear upon it, no movement of any-
kind takes place : volition is wholly absent.

If, however, one of the toes be gently pinched, the leg
is immediately drawn up close to the body.

2S8 ELEMENTARY PHYSIOLOGY. [less.-

If the skin between the thighs around the anus be
pinched, the legs are suddenly drawn up and thrust out
again violently.

If the flank be very gently stroked, there is simply a
twitching movement of the muscles underneath ; if it be
more roughly touched, or pinched, these twitching move-
ments become more general along the whole side of the
creature, and extend to the other side, to the hind legs,
and even to the front legs.

If the digits of the front limbs be touched, these will be
drawn close under the body as in the act of clasping.

If a drop of vinegar or any acid be placed on the top of
one thigh, rapid and active movements will take place in
the leg. The foot will be seen distinctly trying to rub
off the drop of acid from the thigh. And what is still
more striking, if the leg be held tight and so prevented
from moving, the other leg will begin to rub off the acid.
Sometimes if the drop be too large or too strong, both legs
begin at once, and then frequently the movements spread
froni the legs all over the body, and the whole animal is
thrown into convulsions.

Now all these various movements, even the feeblest and
simplest, require a certain combination of muscles, and
some of them, such as the act of rubbing off the acid, are
in the highest degree complex. In all of them, too, a cer-
tain purpose or end is evident, which is generally either to
remove the body, or part of the body, from the stimulus,
from the cause of irritation, or to thrust away the offending
object from the body : in the more complex movements
such a purpose is strikingly apparent.

It seems, in fact, that in the frog's spinal cord there are
sets of nervous machinery destined to be used for a variety
of movements, and that a stimulus passing along a sensory
nerve to the cord sets one or the other of these pieces of
machinery at work.

14. Thus one important function of the spinal cord is
to serve as an independent nervous centre, capable of
originating combined movements upon the reception of
the impulse of an afferent nerve, or rather, perhaps, a group
of such independent nen'ous centres.

But the spinal cord has another most important function,
that of transmitting nervous impulses between the brain

XL] CONDUCTION OF IMPULSES. 289

and the various organs, such as the muscles and the skin,
with which the spinal ner\es are connected. When we
move a foot, certain nervous impulses, starting in some
part of the cerebral hemispheres, pass down along the
whole length of the spinal cord as far as the roots of the
spinal nerves going to the legs, and issuing along the
fibres of the anterior bundles of these roots find their way
to the muscles which move the foot. Similarly, when the
sole of the foot is touched, afterent impulses travel in the
reverse way upward along the spinal cord to the brain.
And the question arises, in what manner do these efferent
and afferent impulses travel along the spinal cord ?

This question is one very difficult to answer, and indeed,
a complete and exact statement is not, at present, possible.
There is, however, a ver}' considerable amount of evidence
which goes to show that both afferent and efferent im-
pulses, on their way between the brain and peripheral
organs, pass chiefly along the longitudinal white fibres of
the cord, especially along those placed in the lateral
columns (§ 4 and Fig. 83). But the afterent impulses
before they get into the lateral columns appear to have to
make their way through a certain quantity of grey matter ;
similarly the efterent impulses when they leave the lateral
columns appear to pass into the grey matter before they
find their way into the anterior nerve roots ; and we shall
see in Lesson XII. that the fibres of the anterior roots are
connected, in a special manner, with the nerve cells of the
anterior cornua. There is also evidence to show that the
grey matter itself may transmit both kinds of impulses, at
least, for a certain distance.

From many experiments it would appear that both
kinds of impulses have a tendency as they travel upwards
or downwards in the spinal cord to cross over from one
side of the cord to the other, and this seems to be
especially the case with the afterent or sensory impulses.
Thus a section of one lateral half of the cord in the dorsal
region aftects both the power of movement and the acute-
ness of the sensations in both legs.

But our knowledge of the way these impulses pass up
and down the cord requires to be enlarged by further
investigations before any very satisfactory statements can
be made about them.

290 ELEMENTARY PHYSIOLOGY. [less.

15. Such are the functions of the spinal cord, taken as
a whole. The spinal nerves are, as we have said, chiefly
distributed to the muscles and to the skin. But other
nerves, such as those for instance belonging to the blood-
vessels, the so-called •J7c?i'^?-w<'?/^^rnerves (Lesson IT § 23),
though many of them run for long distances in the sym-
pathetic system, may ultimately be traced to the spinal
cord. Along the spinal column the spinal nerves give off
branches which run into and join the sympathetic system.
And the vaso-motor fibres which run along in the sym-
pathetic nerves do really spring from the spinal cord,
finding their way into the sympathetic system through
these communicating or commissural branches. Besides
which, some vaso-motor fibres run in spinal nerves along
their whole course.

Experiments moreover go to show that the nervous in-
fluences which, through these vaso-motor nerves, regulate
the blood-vessels, now forcibly constricting them, now
allowing them to dilate, and now keeping them in a state
of moderate or tonic constriction, proceed from the spinal
cord.

The cord is, therefore, spoken of as containing centres
for the vaso-motor nerves or, more shortly, vaso-?notor
centres.

For example, the muscular walls of the blood-vessels
supplying the ear and the skin of the head generally, are
made to contract, as has been already mentioned, by
nervous fibres derived immediately from the sympathetic.
These fibres, however, do not arise from the sympathetic
ganglia, but simply pass through them on their way from
the spinal cord, to the upper dorsal region of which they
can all be traced. At least, this is the conclusion drawn
from the facts, that irritation of this region of the cord
produces the same effect as irritation of the vaso-motor
nerves themselves, and that destruction of this part of the
cord paralyses them.

It has, however, been further shown that the nervous
influence does not originate here, but proceeds from
higher up, from the medulla oblongata in fact, and simply
passes down through this part of the spinal cord on its
way to join the sympathetic nerves,

16. The brain (Fig. 84) is a complex organ, consisting

XI.]

2gt

Fig. 84. — The Base of the Brain.

A. frontal lobe ; B. temporal lobe of the cerebral hemispheres ; Ci. cerebel-
lum ; /. the olfactory nerve ; //. the optic nerve ; ///. /K. lY. the nerves
of the muscles of the eye ; y. the trigeminal nerve ; V/f. the portio dura ;
Via. the auditor>^ nerve ; IX. the glossopharyngeal ; X. the pneumo-
gastric ; XI. the spinal accessory ; XII. the hypoglossal, or motor nerve
of the tongue. The number VI . is placed upon the pons Varolii. The
crura cerebri are the broad bundles of fibres which lie between the third
and the fourth nerv'es on each side. The medulla oblongata (^/} is seen to
be really a continuation of the spinal cord ; on the lower end are seen the
two crescents of grey matter ; the section, in fact, has been carried through
the spinal cord, a little below the proper medulla oblongata. From the
sides of the medulla oblongata are seen coming off the X., XI., and XII.
nerves ; and just where the medulla is covered, so to speak, by the trans-
versely di.sposed />ofts Varolii, are seen coming off the VII. nerve, and
more towards the middle line the VI. Out of the substance of the pons
springs the V. nerve. In front of that is seen the well-defined anterior

U 2

292 ELEMENTARY PHYSIOLOGY. [less.

of several parts, the hindermost of which, termed medulla
oblongata^ passes insensibly into, and in its lower part has
the same structure as, the spinal cord.

Above, however, it widens out, and the central canal,
spreading with it, becomes a broad cavity, which (leaving
certain anatomical minutiae aside) may be said to be
widely open above. This cavity is termed the fourth
ventricle. Overhanging the fourth ventricle is a great
laminated mass, the cerebellum {Cb. Figs. 84, 85, 86). On
each side, this organ sends down several layers of trans-
verse fibres, which sweep across the brain and meet in the
middle line of its base, forming a kind of bridge (called
pons Varolii, Fig. 84) in front of the medulla oblongata.
The longitudinal nerve-fibres of the medulla oblongata
pass forwards, among, and between these layers of trans-
verse fibres, and become visible, in front of the pons, as
two broad diverging bundles, called crura cerebri (Fig.
84). Above the crura cerebri lies a mass of ner\-ous
matter raised up into four hemispherical elevations, called
corpora guachigenii/ia {C.Q. Fig. 86). Between these
and the crura cerebri is a narrow passage, which leads
from the fourth ventricle into what is termed the t/iird
ventricle of the brain. The third ventricle is a narrow
cavity lodged between two great masses of nervous
matter, called optic thalanii, into which the crura cerebri
pass. The roof of the third ventricle is merely membra-
nous ; and a peculiar body of unknown function, the
pineal body, is connected with it. The floor of the third
ventricle is produced into a sort of funnel, which ends in
another anomalous organ, \^^ pituitary body {Pt. Fig. 86 ;
P^ Fig. 84).

The third ventricle is closed, in front, by a thin layer
of nervous matter ; but, beyond this, on each side, there
is an aperture in the boundary- wall of the third ventricle

border of ih^fons; and coming forward in front of that line, between the
JV. and ///. nerves on either side, are seen the crura cerebri. 1 he two
round bodies in the angle between the diverging crura are the so-called
corpora albicantia, and in front of them is P, the pituitary body. This
rests on the chiasma, or junction, of the optic nerves ; the continuation of
each nerve is seen sweeping round the crura cerebri on either side. Im-
mediately in front, between the separated frontal lobes of the cerebral
hemispheres, is seen the corpus callosum, CC. The fissure of Sylvius, about
on a level with /. on the left and //. on the right side, marks the divisioa
between frontal and temporal lobes.

XI.]

THE BRAIN.

293

which leads into a large cavity. The latter occupies the
centre of the cerebral hemisphere^ and is called the lateral.

Fig. 85.
A side view of the brain and upper part of the spinal cord in place — the
parts which cover the cerebro-spinal centres being removed. C. C. the
convoluted surface of the right cerebral hemisphere: Cb. the cerebellum;
M.Ob, the medulla oblongata; B. the bodies of the cervical vertebrcB ;
Sp. their spines ; N. the spinal cord with the spinal nerves.

vejitricle. Each hemisphere is enlarged backwards,
downwards and forwards, into as many lobeC'j aad the

294 ELEMENTARY PHYSIOLOGY. [less.

lateral ventricle presents corresponding prolongations, or
cornua.

The floor of the lateral ventricle is formed by a mass of
nervous matter, called the corpus striatiwi, into which the
fibres of the crura cerebra that have passed by or traversed
the optic thalamus enter (Fig. 86, C.S.).

The hemispheres are so large that they overlap all the
other parts of the brain, and, in the upper view, hide
them.

Their applied faces are separated by a median fissure
for the greater part of their extent ; but, inferiorly, are
joined by a thick mass of transverse fibres, the corpus
callosum (Fig. 84, CC).

The outer surfaces of the hemispheres are marked out
into cojivolutio7ZS, or gyri^ by numerous deep fissures (or
sulci) ^ into which the pia mater enters. One large and
deep fissure which separates the anterior from the middle
division of the hemisphere is called X^o. fissure of Sylvius

(Fig. 84).

17. In the medulla oblongata the arrangement of the
white and grey matter is substantially similar to that
which obtains in the spinal cord ; that is to say, the white
matter is external and the grey internal ; but the grey
matter, containing, as in the spinal cord, nerve cells, is
more abundant than in the spinal cord, and the arrange-
ments of white and grey matter become much more
intricate and complex.

In the brain above the medulla oblongata there are
internal deposits of grey matter, containing nerve cells, at
various places, more especially in the pons Varolii, the
crura cerebri, the corpora quadrigemina, optic thalami
and corpora striata. And there is a remarkably shaped
deposit of grey matter in the interior of the cerebellum,
on each side. But what especially characterizes the brain
is the presence of grey matter of a special nature, con-
taining peculiarly shaped nerve cells, on the surface of the
cerebral hemispheres, and on that of the cerebellum.
This superficial grey matter covers the whole surface of both
these organs, dipping down into the fissures (sulci) of the
former, and following the peculiar plaits or folds into
which the latter is thrown.

The fibre? constituting the white matter of the brain

XI.J

CRANIAL NERVES.

295

and connecting the various deposits of grey matter with
each other and with the spinal cord, are arranged in a
ver)' comphcated manner.

18. Ner\-es are given off from the brain in pairs, which
succeed one another from before backwards, to the num-
ber of twelve (Fig. 86). These are often called " cranial"
ner\-es, to distinguish them from the spinal nerves.

Fig. S6. — A Diagram illustrating the Arrangement of the Parts
OF THE Brain and the Origin of the Nerves.

H. the cerebral hemispheres ; C.S. corpus striatum ; Tk. optic thalamus ;
P. pineal body ; Pi. pituitary body ; C.Q. corpora quadrigemina ; Cb. cere-
bellum : M. medulla oblongata ; /. — All. the pairs of cerebral nerves ;
.5"/. I, S/>, 2, the first and second pairs of spinal nerves.

Tht Jirst pair, counting from before backwards, are the
olfactory nerves., and the second are the optic nen^es. The
functions of these have already been described.

The third pair are called niotores oculi (movers of the
eye), because they are distributed to all the muscles of the
eye except two.

The nen-es of the fourth paif and of the sixth pair
supply, each, one of the muscles of the eye, on each side ;
the fourth going to the superior oblique muscle^ and the

296 ELEMENTARY PHYSIOLOGY. [less.

sixth to the external rectus. Thus the muscles of the eye,
small and close together as they are, receive their nervous
stimulus by three distinct nerves.

Each nerve of the fifth pair is very large. It has two
roots, a motor and a sensory, and further resembles a
spinal nerve in having a ganglion on its sensory root. It
is the nerve which supplies the skin of the face and the
muscles of the jaws, and, having three chief divisions, is
often called trigeminal. One branch containing sensory
fibres supplies the fore-part of the mucous membrane of
the tongue, and is often spoken of as the gustatory.

The seveiith pair furnish with motor nerves the muscles
of the face, and some other muscles, and are called
facial.

The eighth pair are the auditory nerves. As the seventh
and eighth pairs of nerves leave the cavity of the skull
together, they are often, and especially by English writers
on anatomy, reckoned as one, divided into portio dicra, or
hard part (the facial) ; and portio mollis^ or soft part (the
auditory) of the " seventh " pair.

The ninth pair in order, the glossopharyngeal^ are
mixed nerves ; each being, partly, a ners'e of taste, and
supplying the hind-part of the mucous membrane of the
tongue, and, partly, a motor nerve for the pharyngeal
muscles.

The tenth pair are the two pneumogastric nerves, often
called the par vagiim. These very important nerves, and
the next pair, are the only cranial nerves which are dis-
tributed to regions of the body remote from the head.
The pneumogastric supplies the larynx, the lungs, the
liver, and the stomach, and branches of it are connected
with the heart.

The elevefith pair again, called spinal accessory^ differ
widely from all the rest, in arising from the sides of the
spinal marrow, between the anterior and posterior roots of
the dorsal nerves. They run up, gathering fibres as they
go, to the medulla oblongata, and then leave the skull by
the same aperture as the pneumogastric and glossopha-
ryngeal. They are purely motor nerves, supplying certain
muscles of the neck, while the pneumogastric is mainly
sensor)^ or at least afferent. As, on each side, the glosso-
pharyngeal, pneumogastric, and spinal accessory nerves

XI.] FUNCTIONS OF MEDULLA OBLONGATA. 297

leave the skull together, they are frequently reckoned as
one pair, which is then counted as the eighth.

The last two nerves, by this method of counting, become
the ninth pair, but they are really the twelfth. They are
the motor nerves which supply the muscles of the tongue.

19. Of these nerves, the two foremost pair do not pro-
perly deser^-e that name, but are really processes of the
brain. The olfactory pair are prolongations of the cere-
bral hemispheres ; the optic pair, of the walls of the third
ventricle ; and it is worthy of remark, that it is only these
two pairs of what may be called false nerves which arise
from any part of the brain but the medulla oblongata or
its immediate vicinity — all the other true nerves being
indirectly, or directly, traceable to that part of the brain,
while the olfactory and optic nerves are not so traceable.

20. As might be expected from this circumstance alone,
the medulla oblongata is an extremely important part of
the cerebro-spinal axis, injury to it giving rise to immediate
evil consequences of the most serious kind.

Simple puncture of one side of the floor of the fourth
ventricle produces for a while an increase of the quantity
of sugar in the blood, beyond that which can be utilized
by the organism. The sugar passes off by the kidneys,
and thus this shght injury to the medulla produces a
temporary disorder closely resembling the disease called
diabetes.

More extensive injury arrests the respiratory processes,
the medulla oblongata being as we have seen (Lesson IV.
§ 24), the nervous centre which gives rise to the contrac-
tions of the respirator}' muscles and keeps the respiratory
pump at work.

And the heart may be stopped, for a time at least,
by irritation of the fibres of the pneumogastric nerve at
their origin in the medulla (see Lesson H. § 27).

We have just seen (§ 15) that the medulla oblongata
acts as an important centre for the vaso-motor nerves.
It is also a nervous centre for the act of swallowing, for
the secretion of saliva, and for many other actions. And
when we remember that every impulse, afferent or efferent,
passing between the higher parts of the brain, and eveiy
nerve of the body, with the exception of the optic, olfac-
tory (and perhaps the third and fourth eye nerves), must

298 ELEMENTARY PHYSIOLOGY. [less.

make its way through some part or other of the me-
dulla oblongata, the importance of this organ becomes
obvious.

21. It is a singular fact that when one side of the brain
is diseased or injured, the effects are visible on the other
side of the body. Thus when, as not unfrequently
happens, a blood-vessel gives way in the right cerebral
hemisphere, leading to a destruction of ner\-ous matter
there, the result is that the left arm, and left leg, and left
side of the body generally are paralysed, that is, the will
has no longer any power to move the muscles of that
side, and impulses started in the skin of that side cannot
awaken sensations in the brain. Hence, it is said that
between the brain and the peripheral organs there is a
complete crossing or decussation of efferent (voluntary)
and afferent (sensory) impulses. We have already seen
(§ 14) that a certain amount of crossing of impulses of
both kinds takes place all along the spinal cord ; but the
chief decussation seems to take place in the medulla
oblongata, and is probably largely, though not wholly,
effected by means of what is called the decussation of the
anterior pyramids (see Fig. 84). Here, large bundles of
fibres coming chiefly from the lateral columns of the
spinal cord (which as we have seen (§ 14) seem to be
the chief channels for the conduction of sensory and
motor impulses along the cord), rise up to the front and
cross over to the other side.

But there is also a decussation of impulses in the case of
the nerves arising from the medulla above the decussation
of the pyramids. Thus, in the case quoted above of a blood
vessel bursting in the right cerebral hemisphere, the left
side of the man's face is paralysed as well as the left side
of his body, that is to say, impulses cannot pass to and
from his brain and the left facial and fifth nerves. The
impulses along these nerves cross over, decussate, and
reach the right side of the brain.

It sometimes happens, however, that disease or injury
may affect the medulla oblongata itself, on one side only
{e.g. the right), above the decussation of the pyramids,
in such a way that the fifth and facial nerves are
affected in their course before they decussate, that is to
say, on the same side as the injury. The man then.

XI.] FUNXTIONS OF CEREBRAL HEMISPHERES. 299

while still paralysed on the left side of his body, is
paralysed on the right side of his face.

22. The functions of most of the parts of the brain
which lie in front of the medulla oblongata are, at present,
very ill understood ; but it is certain that extensive
injur}', or removal, of the cerebral hemispheres puts an
end to intelligence and voluntary movement, and leaves
the animal in the condition of a machine, working by the
reflex action of the remainder of the cerebro-spinal axis.

We have seen that in the frog the movements of the
body which the spinal cord alone, in the absence of the
whole of the brain, including the medulla oblongata, is
capable of executing, are of themselves strikingly complex
and varied. But none of these movements arise from
changes originating within the organism, they are not
what are called voluntary or spontaneous movements ;
they never occur unless the animal be stimulated from
without. Removal of the cerebral hemispheres is alone
sufficient to deprive the frog of all spontaneous or volun-
tary movements ; but the presence of the medulla oblongata
and other parts of the brain (such as the corpora quadri-
gemina, or what corresponds to them in the frog, and the
cerebellum) renders the animal master of movements of a
far higher nature than when the spinal cord only is left.
In the latter case the animal does not breathe when
left to itself, lies flat on the table with its fore-limbs
beneath it in an unnatural position ; when irritated kicks
out its legs, and may be thrown into actual convulsions,
but never jumps from place to place ; when thrown into
a basin of water falls to the bottom like a lump of lead,
and when placed on its back will remain so, without
making any effort to turn over. In the former case the
animal sits on the table, resting on its front limbs, in
the position natural to a frog ; breathes quite naturally ;
when pricked behind jumps away, often getting over a
considerable distance; when thrc^wn into water begins at
once to swim, and continues swimming until it finds some
object on which it can rest ; and when placed on its back
immediately turns over and resumes its natural position.
Not only so, but the following ver)' striking experiment
may be performed with it. Placed on a small board it
remains perfectly motionless so long as the board is

300 ELEMENTARY PHYSIOLOGY. [less.

horizontal ; if, however, the board be gradually tilted up
so as to raise the animal's head, directly the board becomes
inclined at such an angle as to throw the frog's centre of
gravity too much backwards, the creature begins slowly
to creep up the board, and, if the board continues to be
inclined, will at last reach the edge, upon which when the
board becomes vertical he will seat himself with apparent
great content. Nevertheless, though his movements when
they do occur are extremely well combined and appa-
rently identical with those of a frog possessing the whole
of his brain, he never moves spontaneously, and never
stirs unless irritated.

Thus the parts of the brain below the cerebral hemi-
spheres constitute a complex nervous machinery for
carrying out intricate and orderly movements, in which
afferent impulses play an important part, though they
do not give rise to clear or permanent affections of
consciousness.

23. There can be no doubt that the cerebral hemi-
spheres are the seat of powers, essential to the production
of those phenomena which we term intelligence and will ;
and there is experimental and other evidence which seems
to indicate a connection between particular parts of the
surface of the cerebral hemispheres, and particular acts.
Thus irritation of particular spots in the anterior part of
a dog's brain will give rise to particular movements of
this or that limb, or of this or that group of muscles ; and
the destruction of a certain part of the posterior lobes of
the cerebral hemispheres is said to cause blindness. But
the exact way in which these effects are brought about is
not yet thoroughly understood ; and even if it should be
ultimately proved beyond all doubt, that the central end-
organ of vision (Lesson VI IL § 28) consists of certain
nerve-cells lying in a particular part of the posterior sur-
face of the cerebral hemisphere, and that the central
end-organ of hearing consists of other nerve-cells lying
elsewhere on the cerebral surface, it will still leave us com-
pletely in the dark as to what goes on in the cerebral
hemispheres when we think and when we will.

There is no doubt that a molecular change in some
part of the cerebral substance is an indispensable ante-
cedent to every phenomenon of consciousness. And it is

XI.] REFLEX ACTIONS OF THE BRAIN. 301

possible that the progress of investigation may enable us
to map out the brain according to the psychical relations
of its different parts. But supposing we get so far as to
be able to prove that the irritation of a particular fragment
of cerebral substance gives rise to a particular state of
consciousness, the reason of the connection between the
molecular disturbance and the psychical phenomenon
appears to be out of the reach, not only of our means of
investigation, but even of our powers of conception.

24. Even while the cerebral hemispheres are entire,
and in full possession of their powers, the brain gives
rise to actions which are as completely reflex as those of
the spinal cord.

When the eyelids wink at a flash of light, or a threatened
blow, a reflex action takes place, in which the afferent
nerves are the optic, the efferent the facial. When a bad
smell causes a grimace, there is a reflex action through
the same motor nerve, while the olfactory nerves constitute
the afferent channels. In these cases, therefore, reflex
action must be effected through the brain, all the nerves
involved being cerebral.

When the whole body starts at a loud noise, the
afferent auditory nerve gives rise to an impulse which
passes to the medulla oblongata, and thence affects the
great majority of the motor nerves of the body.

25. It may be said that these are mere mechanical ac-
tions, and have nothing to do with the operations which
we associate with intelligence. But let us consider what
takes place in such an act as reading aloud. In this case,
the whole attention of the mind is, or ought to be, bent
upon the subject-matter of the book ; while a multitude of
most delicate muscular actions are going on, of which the
reader is not in the slightest degree aware. Thus the book
is held in the hand, at the right distance from the eyes ;
the eyes are moved from side to side, over the lines and
up and down the pages. Further, the most delicately
adjusted and rapid movements of the muscles of the
lips, tongue, and throat, of the lar}-ngeal and respirator)'
muscles, are involved in the production of speech. Per-
haps the reader is standing up and accompanying the
lecture with appropriate gestures. And yet every^ one
of these muscular acts may be performed with utter

302 ELEMENTARY PHYSIOLOGY. [less.

unconsciousness, on his part, of anything but the sense of
the words in the book. In other words they are reflex
acts.

26. The reflex actions proper to the spinal cord itself
are natm-al^ and are involved in the structure of the cord
and the properties of its constituents. By the help of the
brain we may acquire an infinity of artificial reflex actions,
that is to say, an action may require all our attention and
all our volition for its first, or second, or third perform-
ance, but by frequent repetition it becomes, in a manner,
part of our organization, and is performed without volition,
or even consciousness.

As everyone knows, it takes a soldier a long time to learn
his drill — for instance, to put himself into the attitude of
" attention " at the instant the word of command is heard.
But, after a time, the sound of the word gives rise to the
act, whether the soldier be thinking of it, or not. There
is a story, which is credible enough, though it may not be
true, of a practical joker, who, seeing a discharged veteran
carrying home his dinner, suddenly called out "Atten-
tion ! " whereupon the man instantly brought his hands
down, and lost his mutton and potatoes in the gutter.
The drill had been thorough, and its effects had become
embodied in the man's nervous structure.

The possibility of all education (of which military drill
is only one particular form) is based upon the existence of
this power which the nervous system possesses, of organ-
izing conscious actions into more or less unconscious, or
reflex, operations. It may be laid down as a rule, which
is called the Law of Association, that if any two mental
states be called up together, or in succession, with due
frequency and vividness, the subsequent production of
the one of them will suffice to call up the other, and that
whether we desire it or not.

The object of intellectual education is to create such
indissoluble associations of our ideas of things, in the order
and relation in which they occur in nature ; that of a moral
education is to unite as fixedly the ideas of evil deeds with
those of pain and degradation, and of good actions with
those of pleasure and nobleness.

27. The sy?npathetic system consists chiefly of a double
chain of ganglia, lying at the sides and in front of the

XL] SYMPATHETIC SYSTEM. 30^

spinal column, and connected with one another, and with
the spinal nerves, by commissural cords. From these
ganglia, nerves are given off which for the most part follow
the distribution of the vessels, but which, in the thorax and
abdomen, form great networks, ox plexuses^ upon the heart
and about the stomach and other abdominal viscera. A
great number of the fibres of the sympathetic system are
derived from the spinal cord ; but others originate in the
ganglia of the sympathetic itself.

By means of the sympathetic nerves the muscles of the
vessels generally, and those of the heart, of the intestines,
and of some other viscera may, as we have seen, be in-
fluenced ; and the influence thus conveyed, it may be
remarked, is generally different to, or even antagonistic to
that which is conveyed to the same organs by the fibres
running in the spinal or cranial nerves. Thus while irrita-
tion of the (cranial) pneumogastric fibres stops the heart,
irritation of the sympathetic fibres going to the heart
increases the beat.

But the influences which thus reach these organs through
the sympathetic nerves, do not seem to originate in the
sympathetic system itself, but to be derived from the
spinal cord or brain. We have seen (§ 14) this to be the
case in reference to vaso-motor nerves, and the same is
true of the sympathetic nerves going to the heart and other
viscera. Whatever may turn out to be the function of
the sympathetic ganglia, there is at present no adequate
evidence that they in any way act as nervous centres,
either of reflex action, or of any other form of nervous
activity.

304 ELEMENTARY PHYSIOLOGY. [less.

LESSON XIL

HISTOLOGY ; OR, THE MINUTE STRUCTURE OF THE
TISSUES.

I. Ix the first chapter (§ ii) attention was directed to
the obvious fact that the substance of which the body of
a man or other of the higher animals is composed, is not
of uniform texture throughout ; but that, on the contrary,
it is distinguishable into a variety of components which
differ ver)' widely from one another, not only in their
general appearance, their colour, and their hardness or
softness, but also in their chemical composition, and in the
properties which they exhibit in the living state.

In dissecting a limb there is no difficulty in distinguish-
ing the bones, the cartilages, the muscles, the nerves
and so forth from one another ; and it is obvious that the
other limbs, the trunk, and the head, are chiefly made up
of similar structures. Hence, when the foundations of
anatomical science were laid, more than two thousand
years ago, these " like '" structures which occur in different
parts of the organism were termed homoio^nera, " similar
parts." In modem times they have been termed '' tissues,"
and the branch of biology which is concerned with the
investigation of the nature of these tissues is called
Histology.

Histolog)' is a ver)' large and difficult subject, and
this whole book might well be taken up with a thorough
discussion of even its elements. But physiology is, in
ultimate analysis, the investigation of the vital properties

XII. 1 THE TISSUES. 3o5

of the histological units of which the body is composed.
And even the elements of physiology cannot be thoroughly
comprehended without a clear apprehension of the nature
and properties of the principal tissues.

2. A good deal may be learned about the tissues
without other aid than that of the ordinary methods of
anatomy, and it is extremely desirable that the student
should acquire this knowledge as a preliminary to further
inquiry. But the chief part of modern histology is the
product of the application of the microscope to the
elucidation of the minute structure of the tissues ; and
this has had the remarkable result of proving that these
tissues themselves are made up of extremely small
homoiomera^ or similar parts, which are primitively alike
in form in all the tissues.

3. Every tissue therefore is a compound structure : a
multiple of histological units, or an aggregation of his-
tological elements ; and the properties of the tissue are the
sum of the properties of its components.

The distinctive character of every fully formed tissue
depends on the structure, mode of union, and vital proper-
ties of its histological elements when they are fully formed.
But each tissue can be traced back to a young or
embryonic condition, in which it has no characteristic
properties, and in which its histological elements are so
similar in structure, mode of union, and vital properties to
those of every other embn.-onic tissue, that our present
means of investigation do not enable us to discover any
difference among them.

4. These embryonic, undifferentiated, histological ele-
ments, of which every tissue is primitively composed, or,
as it would be more correct to say, which . in the embryonic
condition, occupy the place of the tissues, are technically
named nucleated cells. The colourless blood corpuscle
(Lesson III. § 6) is a typical representative of such a cell.
And it is substantially correct to say (i) that the his-
tological elements of even.- tissue are modilications or
products of such cells ; (2) that ever}- tissue was once a
mass of such cells more or less closely packed together ;
and (3) that the whole embryonic body was at one time
nothing but an aggregation of such cells.

5. The body of a man or of any of the higher animals

3o6 ELEMENTARY PHYSIOLOGY. [less.

in fact commences as an ovum or egg. This (Fig. 87) is
a minute transparent spheroidal sac, '-,^0 of an inch in
diameter in man, which contains a similarly spheroidal
mass of protoplasm, in which a single large nucleus is
imbedded.

The first step towards the production of all the complex
organization of a mammal out of this simple body is the
division of the nucleus into two new nuclei which recede
from one another, while at the same time the protoplasmic
body becomes separated, by a narrow cleft which runs
between the two nuclei, into two masses, or blastomeres,
(Fig. 88) one for each nucleus. By the repetition of the
process the two blastomeres give rise to four, the four to
eight, the eight to sixteen, and so on, until the embryo is

Fig. 87. — Diagram of the Ovum.

«, Granular protoplasm ; b, nucleus, called " germinal vesicle ;" c, nucleolus,

called "germinal spot."

an aggregate of numerous small blastomeres, or nucleated
cells. These grow at the expense of the nutriment supplied
from without, and continue to multiply by division ac-
cording to the tendencies inherent in each until, long
before any definite tissue has made its appearance, they
build themselves up into a kind of sketch model of the
developing animal, in which model many of, if not all
the future organs are represented by mere aggregates of
undifferentiated cells.

6. Gradually, these undifferentiated cells become
changed into groups or sets of differentiated cells, the cells
in one set being like each other, but unlike those of other
sets. Each set of differentiated cells constitutes a " tissue,"
and each tissue is variously distributed among the several

XII.]

THE TISSUES.

307

organs, each organ generally consisting of more than one
tissue.

And this differentiation in structure is accompanied by
a change of properties. The undifferentiated cells are,
as far as we can see, alike in function and properties as
they are alike in structure. But coincident with their
differentiation into tissues, a division of labour takes place,
so that in one tissue the cells manifest special properties

Fig. 88.— The svccessive division of the Mammalian- Ovum into
Blasto.meres. Somewhat diagrammatic
a, division into vno ; b, into four ; c, into eight, and d, into several blasto-
raeres. The clear ring seen in each case is the zona fellucida^ or

membrane investing the o\Tim.

and earn- on a special work ; in another they have other
properties, and other work ; and so on.

The principal tissues into which the undifferentiated
cells of the embn'o become differentiated, and which are
variously built up into the organs and parts of the adult
body, may be arranged as follows.

(i.) The most important tissues are the w«j^«/ar and

X 2

3o8 ELKMKNTARY PHYSIOLOGY. [less.

nervous tissues, for it is by these that the active life of the
individual is carried on.

(2.) Next come the epithelial tissues, which, on the one
hand, afford a covering for the surface of the body as well
as a lining^ for the various internal cavities of the body :
and, on the other hand, carry on a great deal of the
chemical work of the body, inasmuch as they form the
essential part of the various glandular organs of the body.

(3.) The remaining principal tissues of the body, namely
the so-called co?inccii7>c tissue, cartilaginous tissue and
osseous or bony tissue, form a group by themselves, being-
all three similar in their fundamental structure, and all
three being, for the most part, of use to the body for their
passive rather than for their active qualities. They
chiefly serve to support and connect the other tissues.

These principal or fundamental tissues are often arranged
together to form more complex parts of the body, which
are sometimes spoken of, though in a different sense, as
tissues. Thus various forms of connective tissue are
built up with some muscular tissue and nervous tissue, to
form the blood-vessels of the body (see Lesson I L), which
are sometimes spoken of as "vascular tissue." So again,
a certain kind of epithelial tissue, known as " epidermis,"
together with connective tissue, blood-vessels and nerves,
forms the skin or tegumentary tissue ; a similar com-
bination of epithelium with other tissues constitutes the
mucous membrane lining the alimentary canal, and also
occurs in the so-called "glandular" tissue.

We may confine our attention here to the principal
tissues properly so-called.

7. Epithelial tissue. A good example of this tissue
is to be found in the epidermis of the skin, which, as we
have seen (Lesson V.), consists of the superficial epidermis
which is non-vascular and epithelial in nature, and of the
deep derma, which is vascular, and is indeed chiefly com-
posed of connective tissue carrying blood-vessels and
nerves. And in all the mucous membranes there is a
similar superficial epithelial layer, which is here simply
called epithelium, and a deep layer, which similarly con-
sists of connective tissue carrying blood-vessels and nerves
and may also be spoken of as derma.

8. If a piece of fresh skin is macerated for some time in

XII.] THK EPIDERMIS. 309

water, it will be easy to strip off the epidermis from the
derma.

The outer part of the epidermis which has been de-
tached Idv maceration will be found to be tolerably dense
and coherent, while its deep or inner substance is soft and
almost gelatinous. Moreover, this softer substance fills up
all the irregularities of the surface of the derma to which
it adheres, and hence, where the derma is raised up into
papilU\% the deep or under surface of the epidermis
presents innumerable depressions into which the papillae
fit, giving it an irregular appearance, somewhat like a net-
work. Hence it used not unfrequently to be called the
network of Malpighi {rete Maipii^hii), after a great Italian
anatomist of the seventeenth century, who first properly
described it. On the other hand, its soft and gelatinous
character led to its being called mucous layer {stratum
iniicosuni). Chemical analysis shows that the firm outer
layer of the epidermis difters from the deep soft part by
containing a great deal of horny matter. Hence this is
distinguished as the horny layer {stratian corneum).

In the living subject the superficial layers of the
epidermis become separated from the lower layers and
the derma, when friction or other irritation produces a
" blister." Fluid is poured out from the vessels of the
derma, and, accumulating between the upper and lower
layers of the epidermis, detaches the latter.

9. The epidermis is constantly growing upon the deep
or dermic side in such a manner that the horny layer
is continually being shed and replaced. The "scurf"
which collects between the hairs and on the whole surface
of the body, and is removed by our daily brushing and
washing, is nothing but shed epidermis. When a limb
has been bandaged up and left undisturbed for weeks, as
in case of a fracture, the shed epidermis collects on the
surface of the skin in the shape of scales and flakes, which
break up into a fine white powder when rubbed. Thus we
"shed our skins'' just as snakes do, only that the snake
sheds all his dead epidermis as a coherent sheet at once,
while we shed ours bit by bit, and hour by hour.

10. What is the nature of the process by which the
epidermis is continually removed.?

If a little of the epidermic scurf is mixed with water

^,io ELEMENTARY PHYSIOLOGY. [less.

and examined under a power magnifyinj^ 300 or 400
diameters, it will seem to consist of nothing but irre-
gular particles of very various sizes and with no definite
structure. But if a little caustic potash or soda is
previously added to the water the appearance will be
changed. The caustic alkali causes the horny substance
to swell up and become transparent ; and this is now
seen to consist of minute separable plates, some of which
contain a rounded body in the interior of the plate,
though in many this is no longer recognisable. In fact,
so far as their form is concerned, these bodies have the
character of nucleated cells, in which the protoplasmic
body has been more or less extensively converted into
horny substance.

Thus the cast-off epidermis in reality consists of more
or less coherent masses of cornified nucleated cells.

There is a yet simpler method of demonstrating this
truth. At the margins of the lips the epidermis is continued
into the interior of the mouth, and though it now receives
the name of epithelium it differs from the rest of the skin in
no essential respect except that it is very thin, and allows
the blood in the vessels of the subjacent derma to shine
through. Let the lower lip be turned down, its surface
very gently scraped with a blunt-edged knife, and the
substance removed be spread out, and covered with a
thin glass, and examined as before. The whole field of
view will then be seen to be spread over with flat irregular
bodies very like the epidermic scales, but more trans-
parent, and each provided with a nucleus in its centre
(Fig. 89).

Since these detached scales are always to be found
on the inner surface of the lip, it follows that they are
always being thrown off.

10. The horny external layer of the epidermis, then,
is composed of coherent cornified flattened cells, which
are constantly becoming detached from the soft internal
layer, and must needs be, in some way, derived from it.
But in what way.' Here microscopic investigation
furnishes the answer. For if the soft layer is pro-
perly macerated it breaks up into small masses of
nucleated protoplasmic substance, that is, into nucleated
cells which in the innermost or deepest part of the layer

xir.]

GRcnVTH OF EriDERMTS.

Ill

are columnar in form, being elongated perpendicularly to
the face of the derma, oni which, they rest, and which in
the intermediate region present transitions in form and
other respects between these and the shed scales.

A thin vertical section of epidermis (Fig. 90) in
undisturbed relation with the subjacent derma, leaves
not the smallest doubt (a) that the epidermis consists of
nothing but nucleated cells, with perhaps an intinitesimal
amount of cementing substance between them ; (d) that
from the deep to the superficial part of the derma, the
cells always present a succession from columnar or sub-
cylindrical protoplasmic forms to flattened completely
cornified forms. And since we know that the latter are

Fig. 89. — Two Epithelial Scales from the Interior of the Motth.
A small nucleus « is seen in each, as \vell as fine granulations in the body of

the plate. The edges of the plates are irregular from pressure. Magnified

about 400 times.

constantly being thrown off, it follows (t) that these
gradations of form represent cells of the deep layer
which are continually passing to the surface, and being
thrown oft' there,

1 1. What is the cause of this constant succession ? To
this question, also, microscopic investigation furnishes a
clear answer. The deeper cells are constantly growing
and then multiplying by a process of division in such a
manner that the nucleus of a cell divides into two new
nuclei, around each of which one half of the protoplasmic
body disposes itself. Thus one cell becomes two, and
each of these grows until it acquires its full size at the
expense of the nutritive matters which exude from the

;i2

ELEMENTARY PHYSIOLOGY.

[Less.

—I

—d

Fig. 90.

Section of skin highly magnified — somewhat diagrammatic, a, homy epi-
"iermis ; b, softer layer, reie MalpigMi ; c, dermis ; d, lowermost vertical
laj-er of epidermic cells ; e, cells lining the sweat duct continuous with
epidermic cells ; A, corkscrew canal of sweat duct. To the right of the
sweat duct the dermis is raised into a papilla, in which the small arterj',
y, breaks up into capillaries, ultimately forming the veins, £.

xii.l HISTOLOr.ICAT. MEASUREMENTS. 313

vessels with which the derma is abundantly supplied ;
such a cell in fact possesses the vital properties of a
primitive embryo cell.

The cells nearer the derma are more immediately and
abundantly supplied with nourishment from the dermal
blood-vessels, and serve as the focus of growth and
multiplication for the whole epidermis ; they are in fact
the progenitors of the superficial cells which, as they are
thrust away by the intercalation of new cells between the
last formed and the progenitors, become metamorphosed
in form and chemical character, and at last die and are
cast off.

And it follows that the epidermis is to be regarded as a
compound organism made up of myriads of cells, each
of which follows its own laws of growth and multiplica-
tion, and is dependent upon nothing save the due supply
of nutriment from the dermal vessels. The epidermis,
so far, stands in the same relation to the derm as does
the turf of a meadow to the subjacent soil.

12. Structures which are rendered clearly distinguishable
only by a magnifying power of 300 or 400 diameters must
needs be very small, and it is desirable that, before
going any further, the learner should try to form a definite
notion of their actual and relative dimensions by com-
parison with more familiar objects. A hair of the human
head of ordinary fineness has a diameter of about ^^jth
(say 0*003) of an inch, or o'o8 mm. (millimetre). The hairs
which constitute the fur of a rabbit, on the other hand,
are very much finer, and the thickest part of the shaft
usually does not exceed njVT)th of an inch, i.e. o"ooi inch
or about o'o25 mm. ; while the fine point of such a hair
may be as little as ^s^^o^h of an inch, about o'ooi mm.,
or even less in diameter.

In microscopic histological investigations the range of
the magnitudes with which we have to do ordinarily lies
between o'l and o'ooi millimetre ; that is to say roughly
between one two hundred and fiftieth and one twenty-five
thousandth of an inch. It is therefore extremely convenient
to adopt, as a unit of measurement, o'ooi millimetre,
called a micro-millimetre, and indicated by the symbol /x,
of which all greater magnitudes are multiples. Thus, if
the extreme point of a rabbit's hair has a diameter of i/x,

3i4 ELEMENTARY PHYSIOLOGY. [less.

the middle of the shaft will be 25/A, and the shaft of a
human head hair Sofi.

Adopting this system, the deep cells of epidermis have
on an average a diameter of 12/x or more, the nuclei of
4/i to 5/x, while the superficial cells are plates of about 25/x,
the nuclei retaining about the same dimensions. The
diameter of a white corpuscle of the blood is about lo/x,
that of a red corpuscle being y^i to Sfx. Hence the deep
cells of the epidermis are rather larger than white blood
corpuscles, and the uppermost ones much larger, at least
in superficial area.

13. The epidermis proper ever\'where presents sub-
stantially the same general characters. Its permeability
to water permits, as we have seen, of the transudation of
the insensible perspiration, and it thus plays the part of
an excretory organ, while, in so far as it continually forms
and throws off cornified cells, it might be said to secrete
horny matter.

But in many parts of the body the excretory functions
of the skin are concentrated and intensified by a very
simple modificatio of the epidermis, which is produced
inwards into saccular or tubular pouches. These are the
so-called cutaneous glands wYnch. are of two kinds — sweat
glands and sebaceous glands.

The swea glands, as we have seen (Lesson V.), are
long tubes, the inner ends of which lie deep in the derma
and are coiled up and surrounded by a rich network of
capillar}- vessels. (See Figs. 31, 33, pp. 121, 123, and
Fig. 90.)

The sebaceous glands have rather the form of short
sacculated pouches ; and the substance of their cells under-
goes chemical metamorphosis, not into homy but into fatty
matter, which, as the cells are thrown off and burst, is
poured out through the neck or duct of the pouch.

14. In other regions the cornified cells are not at once
thrown off in flakes, but are at first built up in definite
structures known as ?iails and hairs, which grow by
constant addition to the surfaces by which they adhere to
the epidermis. In the case of the nails, the process of
growth has no limit, and the nail is kept of one size simply
by the wearing away of its oldest or free end. In the
case of the hairs, on the contrary, the growth of each hair

XII.1

HAIRS AND NAILS.

315

is limited, and when its term is reached the hair falls out
and is replaced by a new hair.

Fig. 91.
A, a longitudinal and vertical section of a nail : a, the fold at the base of
the nail ; b, the nail ; c. the bed of the nail. The figure B is a transverse
section of the same — a, a small lateral fold of the integument ; b, nail ;
c, bed of the nail, with its ridges. The figure C is a highly-magnified view
of a part of the foregoing — c, the ridges ; d, the deep layers of epidermis ;
e, the horny scales coalesced into nail substance. (Figs. A and B magnified
about 4 diameters ; Fig. C magnified about 200 diameters.)

15. Underneath each nail the deep or derviic layer of
the integument is peculiarly modified to form the bed of

3l6

ELEMENTARY PHYSIOLOGY. [less.

Fig. 92.— a Hair in its Hair-Sac

'''uofleLl^MV^r^t. '^'"/ ^'' ""^""'^i substance of the shaft, the medulla
not being Msible, c, newest portion of hair growing on the papilla (/)•
d cuticle of hair; e, cavity of hair-sac ; /, epidermis (and root sheaths)
of the harr-sac corresponding to that of the integument (;«) ; ^, division
between dermis and epidermis ; A, dermis cf ha.r-sac correspondfng to der-
"f1nfegum?ir°'^^^' ^' "'"'^ of sebaceous glands; n, hSrny epidermL

///^ muV. It is very vascular, and raised up into numerous
parallel ridges, like elongated papillae (Fig. 91, B, C).

XII.]

HAIRS

317

The surfaces of all these are covered with growing
epidermic cells, which, as they flatten and become con-
verted into horn, form a solid continuous plate, the
nail. At the hinder part of the bed of the nail the
integument forms a deep fold, from the bottom of which,
in like manner, new epidermic cells are added to the base
of the nail, which is thus constrained to move forward.

The nail, thus constantly receiving additions from below
and from behind, slides forwards over its bed, and projects
beyond the end of the finger, where it is worn away or
cut off.

16. A Jiair^ like a nail, is composed of horny cells ;
but instead of being only partially sunk in a fold of

Fig. 93.

Part of the shaft of a hair inclosed within its root-sheaths and treated with
caustic soda, which has caused the shaft to become distorted. — a, medulla ;
b, cortical substance; c, cuticle of the shaft; from dlof, the root-sheaths,
in section. (Magnified about 200 diameters.)

the integument, it is at first wholly enclosed in a kind of
bag, the hair-sac, from the bottom of which a papilla (Fig.
92 z), which answers to a single ridge of the nail, arises.
The hair is developed by the conversion into horn, and
coalescences into a shaft, oi the superficial epidermic cells
coating the papilla. These coalesced and cornified cells
being continually replaced by new growths from below,
which undei^o the same metamorphosis, the shaft of the
hair is thrust out until it attains the full length natural to it.
Its base then ceases to grow, and the old papilla and sac
die away, but not before a new sac and papilla have been
formed by budding from the sides of the old one. These
give rise to a new hair. The shaft of a hair of the head
consists of a central pith, or medullary matter, of a loose

3i8 ELEMENTARY PHYSIOLOGY. [less.

and open texture, which sometimes contains air ; of a
cortical substance surrounding this, made up of coalesced
elongated horny cells ; and of an outer cuticle^ composed
of flat horny plates, arranged transversely round the shaft,
so as to overlap one another by their outer edges, like
closely-packed tiles. The superficial epidermic cells of
the hair-sac also coalesce by their edges, and become
converted into root-sheaths^ which embrace the root of the
hair, and usually come away with it when it is plucked
out.

17. The mucous membrane lining the alimentary
canal, as has been stated, is framed on the plan of the
skin, inasmuch as it consists of a vascular derma, and a
non-vascular epithelium, the latter being composed of cells
in juxtaposition. But except in the region of the mouth,
where as we have seen the epithelium, like the epidermis,
is composed of many layers of cells, arranged as a soft
Malpighian layer and a hard corneous layer, and the
oesophagus where the structure is similar, the epithelium
of the alimentar)' canal and the continuations of that
epithelium into the ducts and alveoli of the various glands,
consists of hardly more than a single layer of cells placed
side by side. Hence in a vertical section of the mucous
membrane the vascular derma is seen to be covered by a
single row of soft nucleated cells ; though sometimes
a second row of inconspicuous small cells may be seen
below the latter. The cells constituting this single layer
van- in shape, being cylindrical or conical or, as especially
in the glands, cubical or spheroidal ; but they always are
delicate masses of protoplasm, each containing a nucleus.
The polygonal hepatic cells (see Lesson V.), are in reality
the epithelium cells belonging to the minute biliary canals
passing between them.

In the trachea and bronchi, the epithelium of the
mucous membrane consists again of several layers of
cells, but all are soft and protoplasmic nucleated masses,
the uppermost layer being cylindrical in form and ciliated.
In the ureter and bladder the epithelium also consists of
several layers of cells which are frequently irregular in
form.

Lastly, the blood-vessels and lymphatic vessels and the
large serous cavities, such as the peritoneal and pleural

XII.] CARTILAGE. 319

cavities, are lined by a peculiar epithelium, different ip
origin from the epithelium of the skin and mucous mem-
branes. It consists of a single layer of flat, nucleated
plates cemented together at their edges. The form of
the plate or cell varies, being sometimes polygonal,
sometimes spindle-shaped, and sometimes quite irregular.

18. A second group of tissues, of which cartilage may
be taken as the simplest form and the type, differs from
epithelium in a very essential feature. In epithelium,
wherever it is found, the cells are placed close together,
and the amount of material existing between the cells or
intercellular material is exceedingly small. In the group
of tissues, however, to which cartilage belongs, a very con-
siderable quantity of intercellular material is, as we shall
see, developed between the individual nucleated proto-
plasmic cells. Hence the cells are, more or less distinctly
imbedded in a substance different from themselves and
called a matrix. In epithelium, though the cells are some-
times joined together by a cement material, this is never
abundant enough to deserve the name of matrix.

19. Cartilage. — Characteristic specimens of this tissue
are to be found in the "sterno-costal cartilages,^' which
unite many of the ribs with the breastbone. A thin but
tough layer of vascular connective tissue invests, and
closely adheres to, the surface of the cartilage. It is
ierm.e.di ihe pericho?idriu?n. The substance of the cartilage
itself is devoid of vessels ; it is hard, but not brittle, for
it will bend under pressure ; and moreover it is elastic,
returning to its original shape when the pressure is re-
moved. It may be easily cut into very thin slices, which
are as transparent as glass, and to the naked eye appear
homogeneous. Dilute acids and alkalies have no effect
upon it in the cold ; but if it is boiled in water, it yields
a substance similar to gelatine, but somewhat different
from it, which is called chondrijie.

The sterno-costal cartilages of an adult man are many
times larger than are those of an infant. It follows that
these cartilages must grow. The only source from whence
they can derive the necessary nutritive material is the
plasma exuded from the blood contained in the vessels of
the perichondrium. The vascular perichondrium therefore
stands in the same relation to the non-vascular cartila-

320 ELEMENTARY PHYSIOLOGY. [less.

g,inous tissue as the vascular derma does to the non-
vascular epidermis. But, since the cartilage is invested on
all sides by the perichondrium, it is clear that no part of
the cartilage can be shed in the fashion that the superficial
layers of epidermis are got rid of As the nutritive materials,
at the expense of which the cartilage grows, are supplied
from the perichondrium, it might be concluded that the
cartilage grows only at its surface. But if a piece of
cartilage is placed in a staining fluid, it will be found that
it soon becomes more or less coloured throughout. In
spite of Its density, therefore, cartilage is very permeable,
and hence the nutritive plasma also may permeate it, and
enable ever}'' part to grow.

20. If a thin section of perfectly fresh and living cartilage
is placed on a glass slide, either without addition or with
only a little serum, it appears to the naked eye, as has
been said, to be as homogeneous as a piece of glass. But
the employment of an ordinary hand magnifier is sufficient
to show that it is not really homogeneous, inasmuch as
minute points of less transparency are seen to be scattered
singly or in groups throughout the thickness of the section.
When the section is examined with the microscope (Fig. 94)
these points prove to be nucleated cells, var}-ing in shape,
but generally more or less spheroidal, sometimes far apart,
sometimes very near, or even in contact with one another,
in which last case the applied sides are flat. Usually
each cell has a single nucleus, but sometimes there are
two nuclei in a cell. And sometimes globules of fat
appear in the protoplasmic bodies of the cells, and may
completely fill them.

As a rule each cell lies in, and exactly fills, a cavity in
the transparent matrix, or tnfercclliilar substance, which
constitutes the chief mass of the tissue. But a pair
of closely opposed flattened cells may occupy only one
cavity, and all sorts of gradations may be found between
hemi-spheroidal cells in contact, and hemi-spheroidal cells
separated by a mere film of intercellular substance, and
widely separate spheroidal, ellipsoidal, or otherwise shaped
cells. In size, the cells vary very much, some being as
small as lO/i, and others as large as 50/i, or even larger.

As the cartilage dies, and especially if water is added
to it, the protoplasmic bodies of the cells shrink and

XII. ]

CARTILAGE.

321

become irregularly drawn away from the walls of the
cavities which contain them, and the appearance of the
tissue is greatly altered.

No structure is discernible in the matrix or intercellular
substance under ordinary circumstances ; but it may be
split up into thin sheets or lamina. The portions of
matrix immediately surrounding the several cavities some-
times differ in appearance and nature from the rest of

Fig. 94. — A Small Portion of a Section of Articular Cartilage
(Frog) very highly magnified (600 diam.)-

J, matrix or intercellular substance ; /, the protoplasmic body of the cartilage
corpuscle ; «, its nucleus, with «', nucleoli ; c, the capsule, or wall of the
cavity in which the cartilage corpuscle lies. The four cells here figured
seem to have arisen from a single cell, by division, first into two and then
into four. The shading of the matrix in an oblique line indicates the
earlier division into two.

the matrix, so as to constitute distinct capsules (Fig. 94, c)
for the cells ; and, at times, the matrix may by appropriate
methods be split up into pieces, each belonging to and
surrounding a cell, or group of cells, and often disposed
in concentric layers.

Close to the perichondrial surface of the cartilage the
cells become smaller and separated by less intercellular

322 ELEMENTARY PHYSIOLOGY. [less.

substance, until at length the transparent chondrigenous
material is replaced by the fibrous collagenous substance
of connective tissue (§ 22), and the cartilage cells take on
the form of " connective tissue corpuscles."

21. In a very young embryo we find in the place of a
sterno-costal cartilage nothing but a mass of closely-applied,
undifterentiated, nucleated cells, having the same essential
characters as colourless blood-corpuscles, or as the deepest
epidermic cells. The rudiment, or embryonic model of
the future cartilage thus constituted, increases in size by
the growth and division of the cells. But, after a time,
the characteristic intercellular substance appears, at first
in small quantity, between the central cells of the mass,
and a delicate sterno-costal cartilage is thus formed.
This is converted into the full-grown cartilage {a) by
the continual division and subsequent growth to full size,
of all its cells, and especially of those which lie at the
surface ; (b) by the constant increase in the quantity of
intercellular substance, especially in the case of the deeper
part of the cartilage.

The manner in which this intercellular substance is
increased is not certainly made out. If the outermost
layer only of each of the protoplasmic bodies of adjacent
cells of the epidermis were to become cornified and fused
together into one mass, while the remainder of each
cell continued to grow and divide and its progeny threw
off fresh outer cornified layers, we should have an
epidermic structure which would resemble cartilage
except that the " intercellular substance " would be
corneous and not chondrigenous. And it is possible that
the intercellular substance of cartilage may be formed
in this way. But it is possible that the chondrigenous
material may be, as it were, secreted by and thrown out
between the cells, as the constituents of the bile are
thrown out between the hepatic cells, or at all events
manufactured in some way by the agency of the cells,
without the substance of the cells being actually trans-
formed into it. Our knowledge will not at present permit
us to form a definite judgment on this point. One thing,
however, seems certain, viz. that the cells are in someway
concerned in the matter ; the matrix is unable to increase
itself in the entire absence of cells.

xir.] CONNECTIVE TISSUE. 323

The embryonic cells, which give rise to cartilage, are
not distinguishable by any means we at present possess
in any respect of importance from those which give rise
to epidermis.

Nevertheless, the common form must disguise a dif-
ferent molecular machinery, inasmuch as the two, when
set going by the conditions of temperature, supply of
oxygen and nutriment to which they are exposed in the
living economy, work out, as their ultimate products,
tissues which differ so widely as cartilage and epidermis.

The embryonic cartilage cells, like the embryonic
epidermic cells, are living organisms in which certain
definitely limited possibilities of growth and metamor-
phosis are inherent, as they are in those equally simple
organisms, the spores of the comnion moulds, Pcnicilliiun
and Mucor. Given the proper external conditions, the
latter grow into moulds of two difterent kinds, while the
former grow into cartilage and horny plates.

22. Connective Tissue (see Lesson I. § 12). — A specimen
of this tissue, taken from the deep surface of the integu-
ment or from between the muscles of a limb, is a soft
stringy substance, which, if a small portion is carefully
spread out in fluid on a glass shde and examined without
the aid of any microscope, is seen to consist of semi-
transparent whitish bands and fibres, of very various
thicknesses, interlaced so as to form a network, the
meshes of which are extremely irregular. .Hence the
older anatomists termed this tissue areolar or cellular.

Boiled in water, the connective tissue swells up and
yields gelatine, which sets into a jelly as the water cools.
After prolonged boiling, especially under pressure it
almost entirely dissolves away into gelatine, only a small
filamentous solid residue remaining behind.

Dilute acids and dilute alkalies also cause connective
tissue to swell up and acquire a glassy transparency, but
they do not dissolve it. For if to a portion of the tissue
thus altered by either acid or alkali, alkali or acid is added
sufficient to neutralise the first, the tissue returns to its
normal condition.

If a specimen thus rendered transparent by dilute
acetic acid is examined with a magnifying glass, fine
dark lines and dots are seen to be scattered through

Y 2

324 ELEMENTARY PHYSIOLOGY. [less.

the apparently homogeneous substance. Placed under
the microscope, the lines are seen to be sharply defined
fibres of a strongly refracting substance. They are very-
elastic and are unaffected by even strong acids or alkalies
or by prolonged boiling. Hence these elastic fibres formed
a considerable part of the residue above mentioned.

The dots seen with the magnifying glass are shown by
the microscope to be small nucleated cells. They are
termed connective iisstie corpuscles^ just as cartilage cells
are called cartilage corpuscles.

Thus, connective tissue resembles cartilage in so far as
it consists of cells separated by a large quantity of inter-
cellular substance ; but this intercellular substance is
soft, areolated, fibrous, and, for the most part, either
collagenous or elastic, in contradistinction from that of
cartilage, which is hard, solid, laminated and chondri-
genous.

A specimen of fresh connective tissue prepared for the
microscope in its own fluid exhibits a very different
appearance. The field of view is occupied by strings or
threads of extremely various thicknesses which cross one
another in all directions and are often waAy. Some of
the threads can be recognised as elastic by their strongly
refracting character, but the majority of them are pale
and not darkly contoured. All the thicker threads and
strings present a fine longitudinal striation as if they were
bundles of extremely fine fibrillas (Fig. 95A). At intervals
such bundles are often encircled by rings of a more re-
fractive substance, and fibres of the like character may be
disposed spirally round the bundles.

When dilute acetic acid is added to the specimen, the
pale threads and longitudinally striated strings swell up
and the longitudinal striation disappears ; hence it is that
the specimen becomes so transparent (Fig. 95B), More-
over it is these striated threads and strings which are
dissolved by boiling water, and yield gelatine. We may
therefore speak of them a$: ij'^/A7^tv/^//j- or gelatine-yielding
fibres, by way of distinction from the fibres of elastic sub-
stance, which do not yield gelatine on boiling, and are of
a different chemical nature.

By various modes of maceration the collagenous fibres
may be resolved into filaments which answer to the space

XII.]

CONNECTIVE TISSUE.

325

between the striae, and are of such extreme fineness
that they may measure less than i/x in diameter. It
would appear therefore that the intercellular substance

Fig. 95. _

A. A small bundle of connective tissue, showing longitudinal fibrillation, and
at a and b encircling (annular, spiral) fibres. Magnified 400 diameters.

B. A similar bundle swollen and rendered transparent by dilute acid. The
encircling fibres are seen at «, a, a.

of the connective tissue in question, is composed of {a)
collagenous filmnents^ united by some cementing sub-
stance into bundles, and of {b) elastic fibres. These latter
are generally united into long meshed networks (Fig. 96).

Fig. 96. — Elastic Fibres of Connective Tissue, forming a loose

NETWORK.

Obtained by special preparation from subcutaneous tissue. Magnified 800

diameters.

With care, the cells or connective iisstie corpuscles also
may be seen even in fresh, living connective tissue (Fig.
97) ; but, as has been stated, they are most distinctly visible

326 ELEMENTARY PHYSIOLOGY. [less.

when the tissue is treated with dilute acetic acid. These
cells, when seen in the fresh tissue, care being taken to
prevent the post-mortem changes which they readily
undergo, are found to be flattened plates almost like
epithelial scales, but with very irregular contours. They
closely adhere to, and are, as it were, bent round the
convex faces of the larger bundles of collagenous fibres.
Besides these Jixed connective tissue corpuscles as they
are called, white blood corpuscles, or lymph corpuscles, or
bodies exceedingly like them, are found lying loose in the
fluid which occupies the meshes of the network of fibres,
and appear to wander or travel through the spaces of
the network by virtue of their power of amoeboid move-
ment (Lesson 111.). Such cells are spoken of as wandering
or migratory cells.

'is.

P'iG. 97. — Two Connective Tissue Corpi'scles.

Each is seen to consist of a protoplasmic branched body, containing a nucleus.

\'erj- highly magnified.

23. Such are the characters of that which may be re-
garded as a typical specimen of connective tissue. But
in different parts of the body this tissue presents great
differences, all of which, however, are dependent upon
the different relative extent to which the various elements
of the tissue are developed.

Thus, (a) The intercellular substance may be ver}- much
reduced in amount in proportion to the cells, as is the
case in the superficial layer of the derma and some
other places.

(d) The intercellular substance is abundant, with the
elastic elements well developed, and the collagenous
elements, with fibrils strongly marked and arranged in
close-set parallel bundles, leaving mere clefts in the place

XII.] VARIETIES OF CONNECTIVE TISSUE. 327

of the wide meshes of ordinary connective tissue. This
structure is seen in such dense and strong forms of con-
nective tissue as Hgaments and tendons.

(c) The elastic element predominates, as in the strong
ligament {Hij^amentuDi nucha') which is so highly developed
in long-necked animals, such as the horse, &c., and in the
chordae vocales of the larynx (see Lesson VII.).

{d) The fibrous or elastic elements abound, but a greater
or less amount of chondrigenous substance is developed
around the corpuscles. These are respectively the fibro-
cartilages and elastic cartilages^ which present every
transition between ordinary cartilage and ordinary con-
nective tissue (epiglottis, intervertebral ligaments). Where
a tendon is inserted into a cartilage, as in the case of the
tendo Achillis, the passage of the cartilage into the tendon
is beautifully displayed. The intercellular substance of
the cartilage gradually takes on the characters of that of
the tendon, and the corpuscles of the cartilage become
connective-tissue corpuscles.

((?) Finally, in many parts of the body fatty matter is
found within the protoplasmic substance of the connective
tissue corpuscles just as we have seen it to be formed in
cartilage corpuscles. The fatty deposit increases in
amount, at the same time distending the body of the cell,
until the latter becomes a spheroidal sac full of fat, with
the nucleus pushed to one side (Fig. 98). The conspicuous
fatty tissue {own^'xTv many parts of the body consists simply
of an aggregation of vast numbers of these modified cells,
held together by a vascular framework furnished by the
connective tissue to which they belong.

24. In a young embryo, the places in which connective
tissue will make its appearance are occupied by masses
of simple undifferentiated nucleated cells. By degrees,
the cells become separated from each other by a trans-
parent intercellular substance or matrix, which eventually
takes on the form of collagenous fibrils and elastic fibres,
the relative proportion and the disposition of the two
varying according to the kind of connective tissue which
is being formed. As in the corresponding case of car-
tilage, the exact part played by the cells in the formation
of this matrix is still a matter of dispute. As the de-
velopment of the tissue proceeds, the cells multiply by

528

ELEMENTARY PHYSIOLOGY.

[less.

division and assume their characteristic flattened and
irregular forms, applying themselves to or rather be-
coming compressed between the bundles of collagenous
fibrils.

Fig. 98. — Adipose Tissue.
Five fat cells, held together by bundles of connective tissueyTi in, the mem-
brane or envelope of the fat cell ; ft, the nucleus, and p, the remains of the
protoplasm pushed aside by the large oil drop a. Magnified 200 diameters.

25. Osseous and dental t/ssi/es. — The substances of
which the bones and teeth are composed present very-
little apparent resemblance to cartilage and connective
tissue, yet they are in reality very closely allied structures.

A fresh long bone, such as the femur or humerus of a

XII.] BONE. 329

rabbit, from which the attached muscles, tendons and
hgaments have been carefully cleaned away, but the
surface of which has not been scraped or otherwise in-
jured, is an excellent subject for the study of bone. It is a
hard tough body which is flexible and highly elastic within
narrow limits, but readily breaks, with a clean fracture, if it
is pressed too far. The two articular ends are coated by a
layer of cartilage which is thickest in the middle. Where
the margins of the cartilage thin out a layer of vascular
connective tissue commences, and extending over the
whole shaft, to the surface of which it is closely adherent,
constitutes \\\^ periosteum. If the bone is macerated for
some time in water, the periosteum may be stripped off in
shreds with the forceps. Filaments pass from its inner
surface into the interior of the bone. If the shaft is
broken across it will be found to contain a spacious
medullary cavity filled by a reddish, highly vascular mass
of connective tissue, abounding in fat cells, called the
medulla or marrow ; and a longitudinal section shows that
this medullary cavity extends through the shaft, but in
the articular ends becomes subdivided by bony partitions
and breaks up into smaller cavities, like the areolae of
connective tissue. These cavities are termed cancelli, and
the ends of the bone are said to have a ca7tce Hated sXxnctnre.
The walls of the medullary cavity in the shaft are very
dense, and exhibit no cancelli and appear at first to be
solid throughout. But on examining them carefully with
a magnifying glass it will be seen that they are traversed
by a meshwork of narrow canals, varying in diameter
from 20|x to loofi or more. The long dimensions of the
meshes lie parallel with the axis of the shaft. These are
the Haversian canals. This system of Haversian canals
opens by short communicating branches on the one hand
upon the periosteal and on the other upon the medullary
surface of the wall of the shaft ; and in a fresh bone,
minute vascular prolongations of the periosteum and of
the medulla respectively, may be seen to pass into the
communicating canals and become continuous with the
likewise vascular contents of the Haversian canals.
Moreover, at one part of the shaft there is a larger canal
through which the vessels which supply the medulla pass.
This is the so-called nutritive for ajneti of the bone. At

330 ELEMENTARY THYSIOLOGY. [less.

the t.vo ends of the bone the cavities of the Haversian
canals open into those of the cancelH ; and the vascular
substance which fills the latter thus further connects the
vascular contents of the Haversian canals with the
medulla.

Thus the bone may be regarded as composed of, a,
an internal, thick, cylinder of vascular medulla ; b, an
external hollow, thin, cylindrical sheath of vascular perios-
teum, completed at each end by a plate of articular
cartilage ; ^, of a fine, regular, long-meshed vascular net-
work which connects the sides of the medullary cylinder
with the periosteal sheath of the shaft ; d, oi z. coarse,
irregular vascular meshwork occupying at each end the
space between the medullar)' cylinder and the plate of
articular cartilage, and connected with the periosteum
of the lateral parts of the articular end ; e, of the hard,
perfect osseous tissue which fills the meshes of these two
networks. Such is the general structure of all long bones
with cartilaginous ends, though some, as the ribs, possess
no wide medullary cavity, but are simply cancellated in
the interior. In some ver}' small bones even the cancelli
are wanting. And there are many bones which have no
connection with cartilage at all.

26. If a bone is exposed to a red heat for some time
in a closed vessel nothing remains but a mass of white
" bone-earth," which has the general form of the bone,
but is ver>' brittle and easily reduced to powder. It
consists almost entirely of calcic phosphate and car-
bonate. On the other hand, if the bone is digested in
dilute hydrochloric acid for some time the calcareous
salts are dissolved out, and a soft, flexible substance is
left, which has the exact form of the bone, but is much
lighter. If this is boiled for a long time it will yield
much gelatin, and only a small residue will be left.
Osseous tissue therefore consists essentially of an animal
matter impregnated with calcic salts, the animal matter
being collagenous like connective tissue, and not chondri-
genous like cartilage.

27. A sufficiently thin longitudinal section made by
grinding down part of the wall of the medullary cavity
of a bone — which has been well macerated in water
and then thoroughly dried — if viewed as a transparent

XII.] BONE. 331

object with a magnifying glass, shows a series of Hnes,
with dark enlargements at intervals, running parallel
with the Haversian canals. If the section, instead of
being longitudinal, were made transversely to the shaft,
and therefore cutting through the majority of the Haver-
sian canals at right angles to their length, similar lines
and dark spots would be seen to form concentric circles
at regular intervals round each Haversian canal (Fig. 99).
The hard bony tissue appears therefore to be composed of
lamellcC, which are disposed concentrically around the
Haversian canals ; and a Haversian canal with the con-
centric lamellas belonging to it form what is called a
Haversian system. The soft substance from which the
bone-earth has been extracted is similarly lamellated, and
here and there presents fibres which may be traced into
the fibrous substance of the periosteum.

If a thin section of dry bone is examined with the
microscope (Fig. 100), by transmitted light, each dark
spot is seen to be a black body (of an average diameter of
about I 5|jl) with an irregular jagged outline, and proceeding
from it are numerous fine dark lines which ramify in
the surrounding matrix and unite with similar branched
lines from adjacent black bodies. The matrix itself has a
somewhat granular aspect. In a transverse section these
black bodies are rounded or oval in form, but in a longitu-
dinal section they appear almost spindle-shaped ; that is
to say they are lenticular or lens-shaped, but flattened as it
were between the adjacent layers of the matrix. Examined
by reflected light the same bodies look white and glisten-
ing ; and if the section instead of being examined dry,
be boiled in water or soaked in strong alcohol, and
brought under the microscope while still wet, the black
bodies with their branching lines will be found to have
almost disappeared, only faint outlines of them being
left. At the same time minute bubbles of air will have
escaped from the section. The black bodies seen in the
dry bone are in fact '* lacuncE^^ i.e. gaps, or holes in the
solid matrix, appearing black by transmitted light and
white by reflected light, because they are filled with air ;
and the dark branched lines are similarly, minute canals,
^^ca?taiicu!i," also filled with air-bubbles, drawn out so to
speak into lines, also hollowed out of the solid matrix, and

332

ELEMENTARY PHYSIOLOGY.

[less.

«*<

^_I5:

.•5?'^ -; ■'fZ = --^x*-

L^-

c/^'^Kr

Fig. 99. — Transverse Section of Compact Bone.
a lamellae concentric with the external surface ; d, lamellae concentric
'with the medullary surface; c, section of Haversian canals ; c*, section of
a Haversian canal just dividing into two ; </, intersystemic lamellae. Low
magnifying power.

placing one lacuna in communication with another. In
each Haversian system the cajialiculi and the lacujicz of

xil]

BONE.

533

the innermost layer or that nearest the Haversian canal
communicate with it, while the canaliculi and the lacuncE
of the outermost layer communicate only with those of
the next inner layer. Hence the laciincz and canaliculi
compose a meshwork of canals, which is peculiar to each
Haversian system, and by which the nutritive plasm

Fig loo. — Transverse Section of Bone, highly magnified (300

diameters).

// Haversian canals ; /, lacunae with canaliculi.

exuded from the vessels in the canal of that system
irrigates all the layers of bone which belong to the
system.

A ver\- thin section of perfectly fresh bone exhibits no
dark bodies, inasmuch as the lacunjE and canaliculi con-
tain no air, but are permeated with the nutritive fluid.
Each lacuna moreover, at all events in young bone,

334 ELEMENTARY PHYSIOLOGY. [less.

contains a nucleated cell, which is altogether similar in
essential character to a connective tissue or cartilage
corpuscle, and if the term were not already misused might
be called a "bone corpuscle." In fact, in ultimate analysis
the essential character of bone shows itself to be this :
that it is a tissue analogous to cartilage and connective
tissue in so far as it consists of cells separated by much
intercellular substance ; and that it differs from them
mainly in the fact that calcareous matter is deposited in
and associated with the intercellular substance in such a
way as to leave mmute uncalcihed passages (the canali-
culi), which open into the larger uncalcified intervals (the
lacuna), in the neighbourhood of the cells.

The function of these passages is doubtless to allow of
a more thorough permeation of the calcified tissue by the
nutritive fluids than could take place if the calcareous
deposit were continuous, and it is probable that, in an
ordinary bone, there is no particle i/i square which is not
thus brought within reach of a minute streamlet of
nutritive plasma.

28. This circumstance enables us to understand that
which one would hardly suspect from the appearance of a
bone, namely, that, throughout life, or, at all events, in
early life, its tissue is the seat of an extremely active
vital process. The permanence and apparent passivity of
the bone are merely the algebraical summation of the
contrary processes of destruction and reproduction which
are going on in it.

If a young pig is fed with madder, its bones will be
found after a time to be dyed red. The madder dye, in
fact, getting into the blood, permanently dyes the tissue
with which it meets in its course through the bones. But
if the pig is fed for a time with madder, and is then
deprived of it, the amount of colour to be found in the
bones depends on the time which elapses before the pig
is killed. And it is not that the colouring matter is
merely, as it were, washed out ; the dye is permanent, but
the bones nevertheless become parti-coloured. In the
shaft of a long bone, for instance, a certain time after
feeding with madder, a deep red layer of bone in the
middle of the thickness of its wall will be found to have
colourless bone on its medullary and on its periosteal

XII.] GROWTH OP BONE. 335

face. And the longer the time which has elapsed since
the feeding with madder, the more completely will the
deep red bone be replaced and covered up by colourless
bone.

Besides, careful inspection of a transverse section of
the wall of the shaft of a long bone is by itself sufficient
to show that bone is constantly being formed and as con-
stantly removed. Such a section exhibits, as has been
said, a number of Haversian canals surrounded by circular
zones formed of concentric layers of bone. But inter-
spersed between these there lie larger and smaller seg-
ments of zones formed of similar concentrically cur^^ed
parallel lamellic, the so-called intersystemic lamellae (Fig.
99, d)^ which have evidently at one time formed parts of
complete Haversian systems, but which have been parti-
ally destroyed and replaced by new systems. In fact, the
formation of new bone is constantly taking place : a^ at
the surface in contact with the periosteum ; ^, at the
surface in contact wdth cartilage ; r, at the surface in
contact with the medulla and its prolongations in the
cancelii and Haversian canals ; and the bone thus
formed is after a time destroyed and replaced by new
growths.

29. To understand this we must study the origin of
osseous tissue. At a certain period of embryonic life
there is no bone in any part of the body. Nevertheless,
the greater number of the "bones," for example the
vertebrae, the ribs, the limb bones, and some of the cranial
and facial bones, exist in a morphological sense, inas-
much that cartilages having the general form of such
bones exist in the places of the future bones. In the
place of the humerus and the femur, for example, there
are rods of pure cartilage, which are, so to speak, small
rough models of the humerus and femur of the adult
When the process of bone formation commences slight
opaque spots, termed " centres of ossificatio7i^' make their
appearance in the substance of the cartilage, the opacity
being due to the deposit of calcareous salts at these
points.

Microscopic examination shows that the calcareous
salts are deposited in the intercellular substance, which,
therefore, is converted into a sort of bone in which the

336 ELEMENTARY PHYSIOLOGY. [less.

lacunae are represented by the cavities of the cartilage
corpuscles. These calcareous salts must reach the centres
of ossification dissolved in the plasma which is exuded
from the perichondrial vessels and permeates the inter-
cellular substance.

In the cartilaginous rudiment of a long bone three
such centres of ossification usually make their appear-
ance, one in the centre of the shaft and one in each end.
Supposing these centres to be formed at the same time
(which may not howevei be the case), what we have to
start from is a rudiment or model in cartilage of the
future bone converted at three points into calcified car-
tilage ; that is to say there is a central nodule {diapliysis)
and two terminal nodules {epiphyses). If the deposit
were to spread from three centres until the three nodules
united the result would be a calcified cartilage in place
of the formative cartilage.

As a matter of fact the deposit does spread through
the rudiment from each centre outwards so long as the
bone is growing. But the cartilage between the diaphysis
and epiphyses and that beyond the ends of the epiphyses
also grow and increase with the general growth of the
bone. That beyond the epiphysial ossification remains
throughout life as articular cartilage, while that between
the epiphysial and diaphysial ossifications is gradually
encroached upon by these and finally obliterated.

If this were all, the adult bone would consist of calcified
cartilage tipped at the ends with cartilage which remained
uncalcified. But this is not all ; such a mass of calcified
cartilage is not a true bone. The adult femur e.g. con-
sists, not of calcified cartilage, but of true osseous tissue
with the characters described above, there being no simple
calcified cartilage anywhere except at the junction of the
articular cartilages with the subjacent bone. And the
true osseous tissue of the femur has a different origin
from that just described, inasmuch as it has been pro-
duced by the calcification in a special way of a peculiar
non-cartilaginous tissue developed from the vascular
sheath of connective tissue surrounding the original
cartilage, which is at first called perichondrium, but
which, as ossification goes on, receives the name of peri-
osteum. This perichondrial or periosteal tissue in a

XII.] OSSIFICATION. 337

somewhat complex manner destroys or absorbs the
calcified cartilage and replaces it by true bone.

In fact, very soon after the ossific centres have made
their appearance, vascular processes of the perichondrium
grow into them. These processes make room for them-
selves by, in some way, causing the destruction and
absorption of the calcified cartilage, giving rise to large
irregular spaces or areolae, which they occupy. The
processes consist of blood-vessels surrounded by a
peculiar form of connective tissue, characterized by the
presence of large nucleated cells called osteoplasts. The
perichondrium or periosteum from which these processes
grow out has a similar structure and is also rich in
osteoplasts.

No sooner have these processes hollowed out the
areola in the calcified cartilage than they begin to line
them with layers of true bone, the matrix of the connec-
tive tissue of the processes being calcified in such a way
as to leave spaces in which some of the cells or osteoplasts
remain imbedded, fine branching canals being left in the
matrix, or being subsequently formed in it. In other words,
layers of true bone, with lacunae containing nucleated
cells and with branched canaliculi, are thus constructed
as a lining to the spaces hollowed out of the calcified
cartilage. None of the spaces, however, are completely
filled up, and there are no signs of regular Haversian
systems with canals and concentric lamincC. The calcified
cartilage is simply replaced by a loose open network of
spongy bone, in the thickness of the bars of which may be
seen the remains of the calcified cartilage, and the cavities
of which are filled with blood-vessels and delicate con-
nective tissue, that is, with marrow.

Meanwhile the perichondrium or periosteum, in addition
to sending in these processes which thus convert the
calcified cartilage into spong}^ but true bone, deposits
layers of somewhat denser but still spong)- bone, on the
outside of the changed and changing ossific centre, in the
form of a cylinder which grows in thickness by the
addition of new layers on its surface, immediately under
the periosteum, and in length by the extension of these
cylindrical layers upwards and downwards. The " peri-
osteal " bone, as this is called, is true bone, the deposition

338

ELEMENTARY PHYSIOLOGY

[less.

of calcic salts taking place in the matrix around the osteo-
plasts in such a way as to leave lacunae and canaliculi.

Very soon after this sheath of periosteal bone has made
its appearance, the spongy bone first formed in the
calcified cartilage is absorbed again by the same vascular

Fig. ioi.— Longitudinal Section of Ossifying Humerus (Dog).
c, the original primitive cartilage ; c, b, spongy bone arising from ossification
of cartilage ; this has already been absorbed and replaced by medulla at
in ; p b_, bone (also spongy) formed by the periosteum ; it is seen extending
as a thin sheet upwards and downwards outside the cartilage. (Magnified
7 diameters.)

processes which formed it, so that soon what was at first
the centre of ossification, after passing from simple
cartilage to calcified cartilage, and so to spongy bone, is
resolved into marrow or medulla, that is into vascular
connective tissue richly loaded with fat.

XII.]

OSSIFICATION.

339

Thus, confining our attention to the diaphysis, we may
say that the primitive femur becomes cut into two halves
by the substitution of vascular medulla for the primitively
non-vascular cartilage. But the cartilage of each half
continues to grow in length and thickness nearest the
medulla, and to be successively converted first into

Fig. I02.— Longitudixal Sectiox of Ossifying Cartilage.

C, region of cartilage ; B, region of bone. _

In C are seen the cartilage cells, c, lying in their cavities, and, arranged in

columns between them, are the bars of calcified matrix c c.
In B are seen the long irregular medullary spaces in, containing the osteoplasts
o and in one is seen a blood-vessel v. These spaces are becoming lined
with true bone, b, in which, as at o , the osteoplasts are entangled, and the
canaliculi visible. Kt c cf are seen the remains of the calcified cartilage,
coated with true bone on each side.

calcified cartilage and then into spongy bone at its end
nearest the medulla.

The two halves, however, are held together by the ring
or cylinder of periosteal bone just described, which grows

z 2

340 ELEMENTARY PHYSIOLOGY. [less.

in thickness and length as the primitive cartilage of the
two halves become more and more separated by calcified
cartilage, spongy bone and medulla. The medulla in-
creases rapidly until the diaphysis assumes the form of a
cylinder of periosteal bone, with narrow but thicker walls
in the middle, and with wider but thinner walls at each
end, somewhat like a long narrow dice-box (Fig. loi).
The middle of the cylinder is occupied by medulla alone,
but each end is, as it were, plugged by a disc of cartilage
undergoing conversion into calcified cartilage, then into
spongy bone, and finally into medulla.

If we take a vertical section of one of these discs
(Fig. 102), we may trace out these changes as they are
taking place.

In the vicinity of its outer face the cartilage cells are
undergoing rapid multiplication, and arrange themselves
in columns parallel with the long axis of the bone, and
therefore perpendicular to the face of the zone of calcified
cartilage. Between these columns the calcified inter-
cellular substance forms partitions, so that the columnar
masses of cells lie in deep honeycomb-Hke chambers with
calcified walls.

Lower down these chambers are seen to be broken into
by vascular processes of the medulla, and converted into
larger irregular chambers, the walls of which are being
lined wtih true bone, containing lacunae and canaliculi.
Still lower down the walls of these new chambers are
seen to be again absorbed, until nothing is left but
medulla.

As the developing bone grows the discs get farther
and farther apart, and the medulla grows longer until the
two ends of the diaphysis meet the epiphyses, and unite
with them. The whole disc thus becomes at last spongy
bone continuous w^ith the similar spongy bone into which
the epiphysis is converted, all that remains of the cal-
cified cartilage being an exceedingly thin layer just below
the articular cartilage at either end of the bone.

Thus though the primitive cartilage serves as the model
of the future bone, a great deal of the bone, namely, the
dense compact bone which forms the shaft and is con-
tinued as a shell over the two ends, does not come from
the cartilage at all but is deposited by the periosteum ;

XII.] OSSIFICATION. 341

the spongy bone at each end is the only part that is
formed in the cartilage, and even in that as we have seen
there are no remains of the cartilage itself.

Moreover the bone even thus formed is subject to
incessant change. The periosteal bone is at first spongy
and slight in texture, and exhibits no true Haversian
systems. Little by little spaces are scooped out in it by
vascular processes of the periosteum on the outside and of
the medulla on the inside, like those which formed it ; and
such a space when formed is in turn filled up in a solid
fashion by layers of bone deposited in a regular way as
concentric lamellas round the blood-vessel of the process,
which in the end remains as the blood-vessel of the
Haversian canal, in the centre of the Haversian system
thus deposited. And indeed similar processes of absorp-
tion and fresh formation go on certainly while the bone
is increasing in size, and probably also for some time
afterwards.

A good many bones, such as the frontal and parietal
bones of the skull, have no cartilaginous precursors. The
roof of the skull of an embryo is formed of connective
tissue, and the primitive centre of ossification in which
one of the bones commences is a calcification of that
part of the connective tissue which occupies the place of
the centre of the future bone. The calcification radiates
from this centre outwards, so that it soon has the
form of a thin plate, the margins of which are as it were
frayed out in filaments. The vascular connective tissue
which incloses the plate becomes its periosteum, and
plays the same part in relation to the growing bone as
the periosteum of cartilage bone does to it. As the
plate grows thicker, medullary processes burrow into it
and give rise to cancelli and Haversian systems.

30. Detital tissues. — The general characters of the teeth
have been given in Lesson VI. § 1 5, Each tooth presents a
crown, which is visible in the cavity of the mouth, where
it becomes worn by attrition with the tooth opposite to it
and with the food ; and one or nxore fanos, which are buried
in a socket furnished by the jawbone and the derma of the
dense mucous membrane of the mouth, which constitutes
\hft gH?n, The line of junction between the crown and
the fang is the fieck of the tooth. In the interior of the

34^

ELEMENTARY PHYSIOLOGY.

[less.

tooth is a cavity communicating with the exterior by-
canals, which traverse the fangs and open at their points.
This cavity is iho. pulp cavity. It is occupied and com-
pletely lilled by a highly vascular tissue richly supplied
with ners-es, the dental pulp, which is continuous below,
through the openings of the fangs, with the vascular
derma of the gum which lies between the fangs and the
alveolar walls, and plays the part of periosteum to both.

Fig. 103.
A, vertical, B, horizontal section of a tooth. — a, enamel of the crown ;
/', pulp cavity ; c, cement of the fangs ; d, dentine. (Magnified about three
diameters.)

31. The tissue which forms the chief constituent of a
tooth is termed dentine (Fig. 103, A, B, ^/). It is a dense
calcified substance containing less animal matter than
bone, and further differing from it in possessing no
lacunae, or proper canaliculi. Instead of these it presents
innumerable, minute, parallel, wavy tubules (Fig. 104^/),
which give off lateral branches. The wider inner ends of
these tubules may measure 4/i or 5/L1 ; they open into the
pulp cavity, while the narrower outer terminations ramify
at the surface of the dentine, and may even extend into
the enamel or cement (Fig. 104).

XII.]

DENTINE.

343

Fig. 104.

A. Enamel fibres viewed in transverse section.

B. Enamel fibres separated and Wewed laterally.

C. A section of a tooth at the junction of the dentine (a) \vith the cement (e) ',

b, c, irregular cavities in which the tubules of the dentine end ; d, fine
tubules continued from them ; /, £■, lacunas and canaliculi of the cement.
(Magnified about 400 diameters.)

The greater part of the crown and almost the -whole of
the fangs consist of dentine. But the summit of the crown

344 ELEMENTARY PHYSIOLOGY. [less.

is invested by a thick layer of a much denser tissue,
which contains only 2 per cent, of animal matter, and is
almost of a stony hardness. This is called enamel (Fig.
103, A, B, (I). It becomes thinner on the sides of the
crown and gradually dies out on the neck. Examined
microscopically, the enamel is seen to consist of six-sided
prismatic fibres (Fig. 104, A. B.) set closely side by side,
nearly at right angles to the surface of the dentine.
These fibres measure not more than 3/4 to 5/x in trans-
verse diameter and present transverse striations.

The third tissue found in teeth is a thin layer of true
bone, generally devoid of Haversian canals, which invests
the outer surface of the fangs and thins out on the neck.
This is termed cement (Fig. 103, A, c ; and Fig. 104, C).

The dental pulp is chiefly composed of delicate con-
nective tissue. It is abundantly supplied with vessels and
nerves, which enter it through the small opening at the
extremity of the fang. The nerves are mainly sensory
branches derived from the fifth pair of cranial nerves.

The superficial part of the pulp, which is everywhere in
immediate contact with the inner surface of the dentine,
consists of a layer of nucleated cells so close set that they
almost resemble an epithelium. They are, however, in
reality connective-tissue cells, and the layer is merely a
slightly modified condition of the stratum of undifferen-
tiated connective tissue, which lies at the surface of
every dermic structure. They are comparable with the
osteoplasts of growing bone, and from them long fila-
mentous processes can be traced into the dentinal tubules.

32. The teeth begin to be developed long before birth,
and while the jaw bones are in a very rudimentary con-
dition. The deep face of the epithelium covering the
free surface of the gum thickens into a ridge, and thus
depresses the corresponding face of the derma, which
at the same time grows up at the sides of the ridge. In
this way a semicircular groove, which is termed the dental
groove, is developed in the derma of the gum of each jaw.
But it must be remembered that the epithelium com-
pletely fills the groove and passes from side to side
smoothly over it. Next, each groove, that in the upper
jaw and that in the lower, becomes subdivided into ten
pouches, five on each side of the middle line, and behind

xii.] DEVELOPMENT OF TEETH. 345

the fifth on each side there remains a residue of the
groove, which may be called a residual pouch.

Each of the first-mentioned pouches becomes gradually
more and more distinct from its neighbours, until at length
its walls unite and shut off the epithelium which it contains
from the cavity of the mouth. The result is a closed bag
full of epithelium, which is a milk tooth sac. At the same
time the derma of the bottom of the sac has grown up
as a conical process into its interior ; and this doital
papilla is the rudiment of the future tooth. It follows
that the epithelium of the sac now forms a thick cap, the
convexity of which is applied to the walls of the sac,
while its concavity fits accurately on the surface of the
papilla.

While the milk-tooth sac is thus shaping itself, its epi-
thelium grows out on one side into a small process, which
gradually increases in size and takes on the characters of
a second tooth sac. This is the sac of the permatioit
tooth, which answers to and will replace each milk
tooth.

A similar change takes place in the residual pouches,
each of which gradually becomes divided into three sacs
for the three hindmost permanent teeth in each jaw.

The sacs of the milk teeth rapidly increase in size and
become separated from one another by partitions of bone
developed from the jaw with which they are in relation,
and which grow up round them. They thus become
lodged in alveoli.

The papilla becomes vascular, and in its central part,
the cells of which it is primitively composed give rise to
connective tissue. At its surface it retains its embryonic
characters, except that the cells become slightly elong-
ated perpendicularly to the surface. These cells, which
are termed odontoplasts, are separated by a delicate
structureless basement membrane from the very similar
cells of the epithelial cap. This is now termed the
enamel organ. It consists of {a) a layer of somewhat
elongated close-set cells adherent to basement mem-
brane covering the papilla, the papillary epitJielijini ;
{b) of a layer of less elongated close-set cells, adherent to
the walls of the sac, the parietal epitheliuni^ continuous
with the papillary epithelium at the base of the papilla ;

346 ELEMENTARY PHYSIOLOGY. [less.

{c) of intermediate cells which have a more or less
stellate form, and adhere loosely.

The proper tooth substance first makes its appearance
as a very thin hollow cap of glassy calcareous deposit at
the summit of the papilla, between the layer of odonto-
plasts and the papillary epithelium. This cap gradually
extends o\"er the whole surface of the papilla (which has
in the meanwhile taken on the form of the future tooth),
and increases in thickness from the summit towards the
base, so that the part of the tooth which is first formed re-
mains, and the new tooth substance can be added only to
its papillary face and to its basal margins. Hence, the
increase of the tooth is accompanied by decrease of the
papilla, which eventually remains in the cavity of the
finished tooth as the pulp. In the region of the crown,
the calcareous deposit which first takes place is extremely
dense, and takes on a prismatic structure ; but in the
deeper layers the deposit takes place in such a manner
as to leave fine interspaces, which become the dentinal
canals. The substance of the pulp has exactly the same
relation to the dentinal canals as the substance of ossi-
fying periosteal or medullary tissue has to the canali-
culi, and a layer of the odontoplasts remains as the layer
of cells mentioned in § 31 as forming the superficial part
of the pulp. The pulp cavity is, as it were, a gigantic
lacuna containing myriads of cells instead of one.

There can be no doubt about the mode of origin of
dentine. But it should be stated that in the opinion of
many the enamel fibres result, not as described above,
from a calcification of the papilla, but from a calcifica-
tion of the cells of the papillary epithelium.

33. The fully formed milk teeth press upon the upper
walls of the sacs in which they are inclosed, and, causing
a more or less complete absorption of these walls, force
their way through. The teeth are then, as it is called,
cut.

The cutting of this first set of teeth, called deciduous^
or ;;////' teeth, commences at about six months, and ends
with the second year. They are altogether twenty in
number — eight being cutting teeth, or incisors ; four, eye
teeth, or canines; and eight, grinders, or molars.

It has been seen that each dental sac of the milk teeth,

XII.] MUSCLE. 347

as it is formed, gives off a little prolongation, which be-
comes lodged in the jaw below the milk tooth, enlarges,
and develops a papilla from which a new tooth is formed.
As the latter increases in size, it presses upon the root of
the milk tooth which preceded it, and thereby causes the
absorption of the root and the final falling out, or shedding,
of the milk tooth, whose place it takes. Thus every milk
tooth is replaced by a tooth of what is termed the perma-
ne7it dentition. The permanent incisors and caiii?ies are
larger than the milk teeth of the same name, but other-
wise differ little from them. The permanent teeth, which
replace the milk molars, are small, and their crowns have
only two points, whence they are called bicuspid. They
never have more than two fangs.

We have thus accounted for twenty of the teeth of
the adult. But there are thirty-two teeth in the complete
adult dentition, twelve grinders being added to the twenty
teeth which correspond with, and replace, those of the
milk set. Permanent back grinders, or molars^ are de-
veloped in the sacs which are formed out of the residual
pouches above mentioned. They have four or five points
upon their square crowns, and, in the upper jaw, commonly
possess three fangs.

The first of these teeth, the anterior molar of each side,
is the earliest cut of all the permanent set, and appears at
six years of age. The last, or hindermost, molar is the
last of all to be cut, usually not appearing till twenty-one
or twenty-two years of age. Hence it goes by the name
of the " wisdom tooth."

34. Muscle {striated). — It is necessary to distinguish
" muscle " as an organ from " muscle " as a tissue.

The biceps muscle, for example (Lesson VIL § 6), is an
organ of a complicated character, of which muscular
tissue forms the predominant constituent only.

As an organ it presents as separate constituents in it,
a., a muscle case ox periinysiiun ; this is a sheath of con-
nective tissue from the inner face of which partitions
proceed and divide the space which it incloses into a
great number of longitudinally disposed compartments ;
^, the muscular fibres which occupy these compartments ;
c^ the vessels which lie in the sheath and in the partitions
between the compartments, and thus surround the mus-

348 ELEMENTARY PHYSIOLOGY. [less.

cular fibres without entering them ; d, the motor nerves
which also at first he in the sheath and in the partitions
between the compartments, but which eventually enter
into the muscular fibres.

T\\^ perimysiu7n forms a complete envelope around the
muscle, which, when it is sufficiently strong to be
dissected off, is known as a fascia ; at each end it
usually terminates in dense connective tissue (/endon),
which becomes continuous with the bone or cartilage to
which the tendon is attached. The partitions given off
from the inner surface of the perimysium form at first
coarse compartments, inclosing large bundles, each con-
sisting of a very great number of fibres. These large
bundles are again divided by somewhat finer connective
tissue partitions into smaller bundles, and these again
into still smaller ones, and so on, the smallest bundles of
all being composed of a number of individual muscular
fibres. In this way the partitions become thinner and
more delicate, until those which separate the chambers
in which the individual muscular fibres are contained
are reduced to little more than as much connective
tissue as will hold the small nerves, arteries and veins
and capillary networks together. As the perimysium
consists of connective tissue, it may be destroyed by
prolonged boiling in water. In fact, in "meat boiled to
rags " we have muscles which have been thus treated ;
the perimysial case is broken up, and the muscular
fibres, but little attacked by boiling water, are readily
separated from one another.

If a piece of muscle of a rabbit which has been thus
boiled for many hours, is placed in a watch-glass with a
little water, the muscular fibres may be easily teased out
with needles and isolated. Such a fibre will be found to
have a thickness of somewhere about 6ofj. (they vary,
however, a great deal), with a length of 30 or 40 milli-
metres, z'.e. about li inch. It is a cylindroidal or
polygonal solid rod, which either tapers or is bevelled off
at each end. By these it adheres to those on each side
of it ; or, if it lies at the end of a series, to the tendon.

The structure and properties of striated muscular
tissue in the histological sense means the structure and
properties of these fibres.

XII.]

MUSCULAR FIBRE

349

^S. The general physical and chemical characters of
muscle and its more conspicuous vital properties have
been already dealt with (Lesson VII. § 4), so that it
remains only to speak of those characters which are
revealed by microscopic investigation.

As we have already had occasion to remark, all tissues
undergo considerable alteration in passing from the living
to the dead state, but, in the case of muscle, the changes

Fig. 105.

A. Part of a muscular fibre (of a frog) seen in a natural condition, d,
dim bands ; l>, bright bands, with the granular line seen in many of them ;
«, nuclei and the granular protoplasm belonging to them, very dimly seen.

B. Portion of prepared mammalian muscular fibre teased out, showing longi-
tudinal portions of variable (i. 2. 3. 4.) thickness ; 4 represents the finest
portion (fibrilla) which could be obtained ; d, dark bands ; i, bright bands,

in the midst of each of which is seen the granular line ^.

which the tissue undergoes in dying, are of such a marked
character that the structure of the dead tissue gives a false
notion of that of the living tissue.

A living striated muscular fibre of a frog or a mammal
is a pale transparent rod composed of a soft, flexible,
elastic substance, the lateral contours of which, when the
fibre is viewed out of the body, appear sharply defined,
like those of a glass rod of the same size ; but when

350 ELEMENTARY PHYSIOLOGY. [less.

the fibre is observed in the living body, bathed in the
lymph which surrounds it, the outlines are not so sharply
defined. In neither case can any distinct line of demarc-
ation between a superficial layer and a deeper substance
be recognised. The fibre appears transversely striped,
as if the clear glassy substance were, at regular intervals
(Fig. 105, A. ^y), converted into ground glass, thus appearing
dimmer. Each of these " dim bands " is about 2fji wide,
and the clear space or " bright band " which separates
every two dim bands is of about the same size, or under
ordinary circumstances somewhat narrower. With a high
power a ver}' thin dark granular line equidistant from each
dim band is discernible in each bright band, dividing the
bright band into two. As these appearances remain when
the object glass is focussed through the whole thickness
of the fibre, it follows that the dim bands, the granular
lines, and the clear spaces on each side of each granular
line, represent the edges of segments of different optical
characters, which regularly alternate through the whole
length of the fibre. Let the excessively thin segments,
of which the thin granular lines represent the edges, be
called g, the thicker, pellucid segments of which the
bright bands on each side of a granular line represent the
edges, B ; and the thickest slightly opaque segments of
which the ground glass like dim bands are the edges, D.
Then the structure of the fibre may be represented by
D. B.g. B. D. B. g. B. indefinitely repeated, and one inch
of length of fibre will contain about 30,000 such seg-
ments, or alternations of structure.

In a perfectly unaltered living fibre the striated sub-
stance presents hardly any sign of longitudinal striation ;
but near to the surface of the fibre in mammalian muscle,
though at various points in the depth of the fibre in the
muscles of the frog, faint indications are to be observed
of the existence of cavities each filled by a nucleus,
surrounded by a small amount of protoplasm (Fig. 105,
A- 11). These are the so-called imiscle corpuscles.

As the muscular fibre dies it undergoes a rapid altera-
tion : — <^, parallel longitudinal striae, often less than 2/x
apart, appear in greater or less numbers until sometimes
the striated substance appears broken up into a mass of
fine delicate fibres ; b, the dim bands become much more

XII.]

MUSCULAR FIBRE.

351

opaque, and hence the transverse striation appears better
marked, until the dim bands may appear like sharply-
defined discs ; r, the nuclei acquire sharp irregular
contours and become much more conspicuous, and c^
especially under certain circumstances and after par-
ticular treatment, a thin superficial layer becomes sharply
separated from the deeper substance of the fibre as a
membrane of glassy transparency, the sa}'colcnii)ia^ which
ensheathes the striated and fibrillated substance.

Fig. 106. — Capillaries of Striated Mlscle.

A. Seen longitudinally. The \\-idth of the meshes corresponds to that of
an ultimate fibre, a, small artery ; b, small vein.

B. Transverse section of striated muscle, a, the cut ends of the ultimate
fibres ; b, capillaries filled \\-ith injection material ; c, parts where the capil-
laries are absent or not filled.

The bright bands and the granular lines, on the other
hand, undergo little alteration.

Under ver)- high powers each granular line looks like
a number of minute granules coherent into an extremely
attenuated plate, the margins of which are attached to
the sarcolemma.

If the sarcolemma of a dead fibre be torn with needles,
the striated substance breaks up in different ways ac-
cording to the treatment to which the fibre has been

352

ELEMENTARY PHYSIOLOGY.

[less.

previously subjected. It may break up into discs, each
of which contains a dim band. Or it may break up
into fibrils, each of which presents the same segmentation
as the whole fibre. These artificial fibrils vary much in
thickness according to mode of preparation and the skill
of the operator ; they may sometimes be obtained of
exceeding fineness (Fig. 105, B.). Transverse sections of
muscular fibre, which have been frozen while perfectly
fresh, present minute close-set circular dots, which ap-
pear to represent the transverse sections of naturally
existing longitudinal fibrils. If the muscle substance is

Fig. 107. — A Muscular Fibre (of Frog) ending in Tendon.

The striated muscular substance, tn, has shrunk from the sarcolemma, s, the

fibrils of the tendon, t, being attached to the latter.

really in this case unaltered the only possible interpre-
tation of the fact is that the fibre is really made up of
fibrils, and that these are invisible in the living muscle
on account of their having the same refractive power
as the interfibrillar substance. But whether the finest
artificial fibrils into which dead muscle may be broken
up are identical with these apparently natural fibrils, it
is not at present certainly determined. In some cases
the artificial fibrils seem smaller than the natural ones,
as if the latter, like the fibre itself, were capable of
longitudinal cleavage.

These are the most important structural appearances

XII.] MUSCULAR FIBRE. 353

presented by ordinary striated muscle. But it may
further be noticed that the dim bands exert a powerful
depolarising influence on polarised light. Hence when a
piece of muscle is placed in the field of a polarising
microscope and the prisms are crossed so that the field
is dark, these bands appear bright. The granular lines
have a similar but very much less marked effect.

36. As in the case of the preceding tissues so in that of
muscle, the place of the adult tissue is occupied in the
embryo by a mass of closely applied, undifferentiated
nucleated cells. As development proceeds, some of these
cells are converted into the tissues of the perimysium,
but others increasing largely in size gradually elongate
and take on the form of more or less spindle-shaped rods
or fibres. Meanwhile the nucleus of each cell repeatedly
divides, and thus each rod becomes provided with many
nuclei, so that each fibre is really a multi-nucleate cell.
Along with these changes the protoplasmic substance of
the original cell becomes, for the most part, converted
into the characteristically striated muscle substance, only
a little remaining unaltered around each nucleus as a
muscle corpuscle.

■^y. The many-nucleated cell metamorphosed into a
muscular fibre is nourished by the fluid exuded from the
adjacent capillaries, and it may be said to respire, inso-
much as its substance undergoes slow oxidation at the
expense of the oxygen contained in that fluid, and gives
off carbonic acid. It is, in fact, like the other elements
of the tissues, an organism of a peculiar kind, having its
life in itself, but dependent for the permanent maintenance
of that life upon the condition of being associated with
other such elementary organisms, through the intermedia-
tion of which its temperature and its supply of nourishment
are maintained.

The special property of a living muscular fibre, that
which gives it its physiological importance, is its peculiar
contractility. The body of a colourless blood corpuscle,
as we have seen, is eminently contractile, insomuch as it
undergoes incessant changes of form. But these changes
take place at all points of its surface, and have no definite
relation to the diameter of the corpuscle, while the
contractility of the muscular fibre is manifested by a

A A

354 ELEMENTARY PHYSIOLOGY. [less.

diminution in the length and a corresponding increase
in the thickness of the fibre. Moreover, under ordinary
circumstances, the change of form is effected very rapidly,
and only in consequence of the application of a stimulus.

When a contracting striated ^.bre is observed under the
microscope all the bands become broader (across the
fibre) and shorter (along the fibre) and thus more closely
approximated. Some observers think that the clear
bands are diminished in total bulk relatively to the dim
bands ; but this is disputed by others. When the fibre
relaxes again the bands return to their previous condition.

38. Non-sfriafed iniisde. — This kind of muscle (also
called plain or smooth muscle) which occurs in the walls
of the alimentary canal, the blood-vessels, the bladder,
and other organs, resembles striated muscle in being
composed of fibres, which are bound together by con-
nective tissue carrying blood-vessels and nerves ; but the

Fig. 108. — A Fibre-cell from the plain, Non-striated Muscular

Coat of the Intestine.

/, granular protoplasm around the nucleus.

non-Striated muscular fibre differs greatly from the striated
fibre. It is very much smaller, being only about 6p. in
width, and from 20|x. to 50^1 in length, and therefore cannot
be seen by the unassisted eye, whereas a large unbroken
striated fibre is visible to a sharp eye. It has only one
nucleus, possesses no sarcolemma, and its substance is
not transversely striated. It is, in fact, a cell which has
become elongated into a flattened spindle, with an oval or
sometimes rod-shaped nucleus in its middle (Fig. 108). A
number of such fibre-cells are united together by a minute
quantity of cement or intercellular substance into a thin
flat band, and a number of such bands are bound to-
gether by connective tissue into larger bands or bundles.
Each fibre is capable of contracting, of shortening into a
thicker oval.

39. Cardiac muscular tissue. — The muscular tissue of
the heart is intermediate in character between striated and
non-striated muscle. Like the non-striated muscle, it is

XII.]

CARDIAC MUSCULAR TISSUE.

composed of cells, each containing a single nucleus, and
possessing no sarcolemma. But the cells (Fig. 109) are
generally short and broad, freciuently branched or irregular
in shape, and their substance is more or less distinctly
striated, like the substance of a striated fibre. A number
of such cells are joined by cement substance into sets of
anastomosing fibres, which are built up in a complex-
interwoven manner into the walls of the ventricles and
auricles.

Fig. log.— Cardiac Fibre Cells.
Two cells isolated from the heart. «, nucleus ; /, line of junction between
the two cells ; p, process joining a similar process of another cell. (Magni-
fied 400 diameters.)

40. Nervous tissue. — The characters of nervous tissue
are very different in different parts of the nervous system.
We may best begin wdth the study of a motor nerve — ■
such an one, for example, as that which supplies the
biceps muscle.

Like the muscle, the nerve is a compound organ consist-
ing of, (^,) a nerve-case or perineufiicui (formerly known
as the neurilemmas)^ partitions from which inclose a
great number of parallel tubular cavities, each of which
contains, [b^ a nerve fibre.

The /c'r/;/cV^r///'w, like the perimysium, is composed of
connective tissue and supports the scanty vessels of the

^ See n3te, p. 356.
A A 2

356 ELEMENTARY PHYSIOLOGY. [less.

nerve. It consists of an external layer, which envelops
the whole nerve, and, within this, layers disposed con-
centrically around, and thus forming secondary sheaths
for, larger and smaller bundles of nerve fibres. Within
these secondary sheaths smaller and smaller groups are
formed until at length partitions, incomplete and of ex-
treme tenuity, are formed between the individual nerve
fibres.

41. The nerve JibTCSy which are the essential elements
of the nerve, vary in diameter from 2/i to i2/x. In the
living state they are very soft cylindrical rods of a glassy,
rather strongly refracting aspect. No limiting membrane
is distinguishable from the rest of the substance of the
rod, but running through the centre of it a band of some-
what less transparency than the rest may be discerned.
At intervals, the length of which varies, but is always
many times greater than the thickness of the rod, the
nerve fibre presents sharp constrictions, which arc termed
7ifldes (Fig. no. B. n n). Somewhere in the interspace
between every two nodes, very careful examination will
reveal the existence of a nucleus (Fig. no, B. tic), in-
vested by more or less protoplasmic substance and lying
in the substance of the rod, but close to the surface.

As the fibre dies, and especially if it is treated with
certain re-agents, these appearances rapidly change.
I. The outermost layer of the fibre becomes recognisable
as a definite membrane, the nciirileniina^ (the so-called
"primitive sheath" or "sheath of Schwann"). 2. The
central band becomes more opaque, and sometimes ap-
pears marked with fine longitudinal strinc as if it were
composed of extremely fine fibrillar ; it is the netiraxis
(" axis cylinder " or "axis fibre" of Remak). 3. \Vherethe
neuraxis traverses one of the nodes the neurilemma is
seen to embrace it closely, but in the intervals between
the nodes a curdy-looking matter, which looks white by.
reflected light, occupies the space between the neuri-\

' This word was formerly used to denote the whole nerve-case, now called
f>eri7teitriufn ; but its similarity to the word sarcoleitivia led to great con-
fusion in the minds of students. It is undoubtedly a wholesome rule never
to use an old word in a new sense ; but the striking similarity between the
two words " neurilemma" and ''sarcolemma," and between the nerve-fibre
sheath and the muscle-fibre sheath, seems an adequate excuse for an exception
to the rule.

XII.]

NERVE FIBRES.

357

Fig. ho. — To Illlstrate the Strlcture of Nerve Fibres.

A. A ner\-e fibre seen without the use of reagents, showing the '' double
contour" due to the medulla, and. n, a node. Neither neurjixis nor neuri'
lemma can be distinctly seen. (Magnified about 300 diameters.)

B. A thin nerve fibre treated with osmic acid, showing, ilc, nucleus with
protoplasm, / surrounding it, beneath the neurilemma ; n n, the two nodes
marking out the segment to which the nucleus belongs. (Magnified 400
diameters.)

C. Portion of fibre (thicker than B), treated mth osmic acid to show the
node 11 ; r,i, the densely stained medulla ; at tn the medulla is seen divided
into segments. (Magnified 350 diameters.)

D. Portion of nerve fibre treated to show the passage of the neuraxis. « x,
through the node, « ; 111, the medulla. At 11 x' the neuraxis is swollen by
the reagents employed and large and irregular. (Magnified 300 diameters.)

E. Portion of nerv'e fibre treated with osmic acid, showing the nucleus, nc,
embedded in the medulla ; r, fine perineurial sheath Ijing outside the
neurilemma, the outline of the latter can only be recognised over the
nucleus «c-. ; the nucleus, nc', belongs to this perineurial sheath. (Mag-
nified 400 diameters.)

F. Portion of nerve fibre deprived of its neurilemma and showing the
medulla broken up into separate fragments, in tn, surrounding th^-
neuraxis, «.r.

358 ELEMENTARY PHYSIOLOGY. [less.

leninia and the ncuraxis. This is the medulla (the so-
called "white substance of Schwann") largely composed
of a complex fatty substance, often spoken of as myelin.
If the neurilemma of a fresh fibre is torn, the myelin
.Hows out and forms irregular lumps as if it were viscous.
The medulla is broken, by oblique lines (Fig. no, C. m)^
extending from the neuraxis to the neurilemma, into seg-
ments, the faces of which are obliquely truncated and fit
closely against one another. These may be seen even in
quite fresh and living nerve fibres. 4. The internodal
nucleus is more sharply defined ; and it will be seen to
be attached to the inner surface of the neurilemma.

The motor nerve, proceeding to its muscle, enters the
perimysium (with which the superficial layer of the peri-
neurium becomes continuous), and divides in the perimysial
septa into smaller and smaller branches, each of which
contains the continuation of a certain number of the fibres
of the nerve trunk, bound up into a bundle by themselves.
In these larger ramifications of the ner\^e trunk there is no
branching of the nerve fibres themselves (at any rate as a
rule), but merely a separation of the compound nerve cord.
In the finest branches, however, the nerve fibres them-
selves may divide ; the division, which always takes place
at a node, is generally dichotomous — that is, one fibre
divides into two, each of these again into two, and so on.
These finest branches consisting of one or two ner\-e fibres,
or of one only, with a very delicate perineurial envelope
(Fig. 1 10, E. c)^ pass to some single muscle fibre, and each
nerve fibre applies itself to the outer surface of the
sarcolemma. At this point, if it has not done so before,
the medulla disappears, the neurilemma becomes con-
tinuous with the sarcolemma, and the neuraxis passes
into a disc of protoplasmic substance containing many
nuclei, which is interposed between the striated muscle
substance and the sarcolemma at this point, thus forming
what is called ?i motor plate or end-plate.^ Before ending
the neuraxis divides and its divisions anastomose freely,
but the exact relations of the various parts of the end-
plate to the muscle-substance have not yet been clearly
made out. The whole appears to constitute an apparatus

' This is the arrangement in most vertebrated animals. In the frog the
neuraxis branches out without entering a distinct motor or end-plate.

XII.] NERVE CELLS. 359

by which the molecular disturbances of the substance
of the neuraxis (the essential part of the nerve) may-
be efficiently propagated to the substance of the muscle.

42. If, instead of following the motor nerve to its distri-
bution in the muscle, we trace it the other way, towards
the spinal cord, we shall find no alteration of any moment
until we arrive at the point at which the anterior root
enters the cord. From the finest branches of the motor
nerve (in which, as has been stated, the nerve fibres
themselves divide) to this point of entry each nerve fibre
extends ensheathed as one continuous undivided neuraxis
in a long succession of internodal segments. At the point
of entry into the cord the perineurium passes into the pia
mater and the general connective tissue framework of
the cord. The neurilemma and the nodes disappear.
Often the neuraxis can be traced towards the anterior
horn of the grey matter, invested only by a sheath of
medulla which gradually becomes thinner and thinner
until at length it disappears, and the fibre, thus reduced
to its neuraxis, passes into one of the processes of one
of the large Jterve cells, which lie in the anterior cornu
of the grey matter (Lesson XL § 5).

These nerve cells are large, the body of the cell having
a diameter varying from 50/^ to loo/x or more. Each
cell, n, contains a large clear nucleus (Fig. in) in which
lies a rounded nucleolus, n' ; the protoplasmic body of
the cell gives off (i) a variable number of ramified pro-
cesses, p, which branch out in all directions into fila-
ments of such extreme tenuity that their terminations
cease to be traceable, and (2) a single simple process, n p,
which becomes continuous with the neuraxis of a motor
nerve fibre.

The neuraxis of a motor nerve fibre, therefore, is an
extremely fine connecting thread or commissure materi-
ally continuous at its central end into a nerve cell, and
at its peripheral end into a muscle cell ; in other words,
these are the central and peripheral end-organs of the
fibre.

43. With one or two exceptions sensory nerve fibres
are not distinguishable by any structural character from
motor nerve fibres. Wherever special-sense organules
exist the sensory fibres are connected with them by

ibo

ELEMENTARY PHYSIOLOGY.

[less.

means of their neuraxis, from which the neurolemma and
medulla have disappeared.

In the case of the spinal nerves the sensory fibres are
collected into the posterior roots, and pass through the
ganglia of those roots. The ganglion consists of nerve
fibres and ner\e cells embedded in a framework of con-
nective tissue which is continuous with the perineurium of
the nerve. Each nerve cell (Fig. 112; consists like a
nerve cell of the spinal cord, of a large nucleus, with a
nucleolus, and of a cell body ; but the cell body is, in most
cases at all events, prolonged into one process only, so

Fig. III. — A large Nerve Cell from the Anterior Cornu of the
SriN.\L Cord.

n, nucleus ; »', nucleolus ; /, branched processes, the fine endings are cut
away ; n/, unbranched process, continued into the neuraxis of a motor fibre.

that the whole cell is pear-shaped. This process sur-
rounded by a neurilemma, which is a prolongation of a
sheath enveloping the cell, soon acquires a medulla, and
thus becomes a ner\'e fibre, which then divides into two
fibres, one of which may be traced into the ner\'e trunk,
and the other along the posterior root to the spinal cord.
Hence the nerve cells of the ganglion appear to be lateral
appendages of the nerve fibres, forming a junction with
them after the fashion of a T-piece. On the central side
of the ganglion the fibres continue their course into the
substance of the spinal cord towards the posterior cornu.

XII.] NON-MEDULLATED NERVE FIBRES.

361

Like the motor ribres they lose their noded neurilemma,
but their ultimate fate is not certainly made out.

44. The fibres just described, whether motor or sensory,
are often spoken of as mcdiillated^ because except at their
peripheral and central terminations they possess the
characteristic medulla. Scattered among these medul-
lated ribres in the spinal and cranial nerves, and ver>'
abundant in the sympathetic nerves, are ribres, which are
often spoken of as non-medullated, because they possess
no medulla. These are pale flattened bands, about as

Fig. 112.— a Nerve Cell from the Ganglion on the Posterior

Root of a Spinal Nerve.
■c. the ner\e cell, with n. nucleus, «'. nucleolus, /, protoplasmic body ; c,
capsule of the ner%e cell ; «", nuclei of the capsule ; n/. the ner\e fibre
which, at the node, d, diN-ides into two. At a the neuraxis of the fibre is
lost in the substance of the cell ; at b it acquires a medulla ; at « " nuclei
are seen on the fibre. At the division the neuraxis d is seen to dh-ide, and
besides the neurilemma. «./., the fibre has an additional sheath, s, con-
tinuous with the capsule of the ner\e cell.

wide as small medullated fibres, often fibrillated longitu-
dinally, and frequently dividing. They appear, in fact, to
be naked neuraxes, without medulla, and apparently with-
out a neurilemma, though they bear at intervals nuclei
which may represent the internodal nuclei of ordinary'
nen.e fibres.

In the sympathetic ganglia are found nerve cells with
several processes, one or more of which may be traced
into such non-medullated fibres.

45. The spinal cord consists of : a^ a connective-tissue

:62

ELEMENTARY PHYSIOLOGY.

[less.

case well supplied with vessels and continuous with the
perineurium of the nerves. This is called the pia viater^
and from it delicate partitions proceed inwards towards
the centre of the cord ; b^ a framework of a peculiar
reticulated modification of connective tissue, termed
neuroglia, which fills up the intervals between the
partitions and bounds the cavities in which, r, the nerve
iibres and nerve cells lie ; and finally of, d, the epithelial
cells lining the central canal, which extends from one end
of the cord to the other.

The brain contains substantially the same elements as
the cord, of which it may be regarded as a sort of ex-
pansion, the ventricles of the brain (all but the fifth)

Fig. 113. — Pale Non-meuullated Fiukes from the Pneumogastric

Nerve.

«, nucleus ; J>, protoplasm belonging to the nucleus.

representing the dilated central canal. The disposition of
the nerve cells and fibres, however, is extremely compli-
cated, and cannot be dealt with here.

Two of the so-called " cranial " nerves require special
notice. That which is commonly called the olfactory
" nerve" is really a lobe of the brain and contains nerve
cells. The proper olfactory nerves are bundles of fibres
which proceed from the under surface of the above and
traverse the cribriform plate to be distributed to the
olfactory mucous membrane. And it is an extremely
remarkable fact that these fibres closely resemble the
non-medullated fibres of the sympathetic nerves, in
being hardly anything more than neuraxes, bearing nuclei

xir.] THE OPTIC NERVE. 363

at intervals. A sheath, apparently representing the neuri-
lemma, is however present in each fibre.

The optic "nerve"' is also properly speaking a lobe of
the brain, and it retains its character as a part of the
central nervous system in so far as its fibres have no
neurilemma and are nodelcss, but it contains no nerve cells
alonsr its course.

APPENDLX.

ANATOMICAL AND PHYSIOLOGICAL CONSTANTS.

The weight of the body of a full-grown man may be taken
at 154 lbs.

I. General Statistics.
Such a body would be made up of —

lbs.

Muscles and their appurtenances 68

Skeleton 24

Skin 10^

Fat 28"

Brain 3

Thoracic viscera 2^

Abdominal viscera 11

147'
Orof—

lbs.

Water 88

Solid matters 66

' The addition of 7 lbs. of blood, the quantity which will readily drain
away from the body, will bring the total to 154 lbs. A considerable quantity
of blood will, however, always remain in the capillaries and small blood-
vessels, and must be reckoned with the various tissues. The total quantity
of blood in the body is now calculated at about i-i3th of the body weight
i.e. about 12 lbs.

366 ELEMENTARY niVSIOLOGY. [append.

The solids would consist of the elements oxygen, hy.
drogen, carbon, nitrogen, phosphorus, sulphur, silicon,
chlorine, fluorine, potassium, sodium, calcium (lithium),
magnesium, iron (manganese copper, lead), and may be
arranged under the heads of —

Proteids. Carbo-hydrates or Amyloids. Fats. Minerals.

Such a body would lose in 24 hours — of water, about
40,000 grains, or 6 lbs, ; of other matters about 14,500
grains, or over 2 lbs. ; among which of carbon 4,000
grains, or more than ^ lb. ; of nitrogen 300 grains ; of
mineral matters 400 grains ; and would part, per diem,
with as much heat as would raise 8,700 lbs. of water from
o^ to V Fahr., which is equivalent to 3,000 foot-tons.'
Such a body ought to do as much work as is equal
to 450 foot-tons.

The losses would occur through various organs, thus

-by

Other
Water. M.\tter. N. C.

grs. grs. grs. grs.

Lungs 5,000 12,000 ... 3,300

Kidneys .... 23,000 1,000 250 140

Skin 10,000 700 10 100

f^a^ces .... 2,000 8co 40 460

40,oco 14,500 300 4,000
The gai7is and losses of the body would be as follows : —

grs.

Creditor— Solid dry food 8,400

Oxygen 10,000

Water 36,100

Total 54,500

grs.

Debtor— Water 40,000

Others Matters 14,500

Total 54,500

' A foot-ton is the equivalent of the work required to lift one ton one foot
high.

APPEND.] CONSTANTS. 367

II. Digestion.

Such a body would require for daily food, carbon 4,000
grains, nitrogen 300 grains.

Now proteids contain, in round numbers, about 15 per
cent, nitrogen, and 50 per cent, carbon, while carbo-
hydrates and fats contain respectively 40 per cent, and 80
per cent, carbon. Hence the necessary amounts of
nitrogen and carbon, together with the other necessary
elements, might be obtained as follows : —

Proteids . . . 2,000 grs. containing: 300 g^rs. nitrogen 1,000 grs. carljon.
CarbO'hydrates 4,500 ,, ,, — ,, 1,800 ,,

Fats .... 1,500 ,, ,. — - ,, 1,200 ,,

Minerals . . 400 ,. ,, — ., —

Water . . . 36,100

44,500 300 4,000

which, in turn, might be obtained, for instance, by means
of—

Pro- Carbo-
TEiDS. Hy- Fats.

grs. DR,\TES.

4,400 grs. very lean meati . -j

containing .... j ^5 P-c proteids . . . 1,100

4,000 grs. bread containing { ^5 P'^; P^^botydrates } ^°° 3.6oo -

3,000 „ potatoes „ {2op:c:Sr°bo'hydrates} ^° ^^^ "

4 p c. proteids . . . \

5 p.c. carb" '---'—'--
4 c.c. fats

6,000 ,, milk ,, ^5 p.c. carbo-hydrates r 240 300 240

1,260 grains of fat, as fat

of meat, butter, > > — — 1,260

dripping, &c. . . . j )

36,500 grs. water — — — —

2 , 000 4, 500 1 . 500

This table, however, must be understood as being
introduced for the sake of illustration only.

The faeces passed, per diem, would amount to about
2,800 grains, containing solid matter 800 grains.

368 ELEMENTARY PHYSIOLOGY. [append.

in. Circulation.

In such a body the heart would beat 75 times a minute,
and probably drive out, at each stroke from each ventricle,
from 5 to 6 cubic inches, or about 1,500 grains of blood.

The blood would probably move in the great arteries at
a rate of about 12 inches in a second, in the capillaries at
I to i^ inches in a minute ; and the time taken up in
performing the entire circuit would probably be about
30 seconds.

The left ventricle would probably exert a pressure on
the aorta equal to the pressure on the square inch of a
column of blood about 9 feet in height ; or of a column of
mercury about 9^ inches in height ; and would do in 24
hours an amount of work equivalent to about 90 foot-tons ;
the work of the whole heart being about 120 foot-tons.

IV. Respiration.

Such a body would breathe about 17 times a minute.

The lungs would contain of residual air about 100 cubic
inches, of supplemental or reserve air about 100 cubic
inches, of tidal air 20 to 2*^ cubic inches, and of comple-
mental air 100 cubic inches.

The vital capacity of the chest— that is, the greatest
quantity of air which could be inspired or expired — would
be about 230 cubic inches.

There would pass through the lungs, per diem, about
350 cubic feet of air.

In passing through the lungs, the air would lose from
4 to 6 per cent, of its volume of oxygen, and gain 4 to 5
per cent, of carbonic acid.

During 24 hours there would be consumed about 10,000
grains oxygen ; and produced about 12,000 grains carbonic
acid, corresponding to 3,300 grains carbon. During the
same time about 5,000 grains or 9 oz. of water would be
exhaled from the respiratory organs.

In 24 hours such a body would vitiate 1,750 cubic feet
of pure air to the extent of i per cent., or 17,500 cubic
feet of pure air to the extent of i per 1,000. Taking the
amount of carbonic acid in the atmosphere at 3 parts, and

APPEND.] CONSTANTS. . 369

in expired air at 470 parts in 10,000, such a body would
require a supply per diem of more than 23,000 cubic feet
of ordinary air, in order that the surrounding atmosphere
might not contain more than i per 1,000 of carbonic acid
(when air is vitiated from animal sources with carbonic
acid to more than i per 1,000, the concomitant impurities
become appreciable to the nose). A man of the weight
mentioned (11 stone) ought, therefore, to have at least
800 cubic feet of well-ventilated space.

V. Cutaneous Excretion.

Such a body would throw off by the skin — of water
about 18 ounces, or 10,000 grains ; of solid matters about
300 grains ; of carbonic acid about 400 grains, in 24
hours.

VI. Renal Excretion.

Such a body would pass by the kid?ieys — of water about
50 ounces ; of urea about 500 grains ; of other solid
matters about 500 grains, in 24 hours.

VII. Nervous Action.

A nervous impulse travels along a nerve at the rate of
about 80 feet in a second in the frog, and of about 100
feet a second in man ; but the rate in man varies very
much according to circumstances.

VIII. Histology.

The following are some of the most important histo-
logical measurements : —

Red blood-corpuscles, breadth ^^-^Xh of an inch, or
7 /x to 8 /i.

White blood-corpuscles, breadth Wijirth of an inch, or
10 \x.

Striated muscular fibre (very variable), breadth ^loth of
an inch, or 60 /x ; length i| inch, or 30 to 40 millimetres.

Non-striated muscular fibre (variable), breadth lEoVath
of an inch, or 6 ft ; length 5 J^y th of an inch, or 50 /u.

E B

370 ELEMENTARV PilVSlOLOGV. [ai-pend.

Nerve fibre (very variable), breadth r^Ucth to 550 oth
of an inch, or 2 /x to 12 /x.

Nerve cells (of spinal cordj excluding processes, breadth
5 0-oth to 55uth or more of an inch, 30 fi to 100 fi or more.

Fibrils of connective tissue, breadth ^rhr^th of an inch,
or I fi.

Superficial cells of epidermis, breadth -oV^jth of an
inch, or 25 /i.

Capillary blood-vessels (variable), width irsVuth to iroVTfth
of an inch, or 7 /x to 12 /x.

Cilia, from the wind-pipe, length ^Tsiiuth of an inch, or

Cones in the yellow spot of the retina, width bu'roth of
an inch, or 3 /x.

I N D E X.

INDEX.

Abdomen {abdo^ I hide), 5
Abdominal aorta, 106
Alnlucrion (ab, from ; duco, I lead),

Absorption {ab, from ; sorbeo. I suck up), from alimentary- canal, 16, io3; 143

blood, 107
intestines, 166
stomach 161

of oxj-gen, 73, 102, 133, 143
water, 167
Accommodation of the ej-e, 253

Acetabulum (a vessel for holding Nnnegar), construction of, 186
Acid, acetic, appearance of blood treated with, 65

connective tissue treated with, 323
carbonic, see Carbonic acid
glycocholic. 129
hydrochloric, calcareous salts dissolved out of bone by, 330

in gastric juice, 157
lactic, 173
taurocholic. 129
uric, 113
Acid reaction of gastric juice, 157, 165

stiffened dead muscle, 172
urine, 112
Acids of the bile. 131
Action, reflex, of the brain. 301

continuance of, in brainless frog, 53, 287, 299
of the spinal cord, 287, 302
in coughing, 97
Acts, particular, connected \\nth particular parts of br^n stirface, 300
'"Adam's apple," 191
Adduction {ad, to, duco, I lead), 1S7
Adipose (adeps. fat), tissue, 32S
Adjustment of the eye, how accomplished, 25S
Aerial waves from sonorous bodies, 232
Afferent and efferent impulses, course of in cord, 289

in medulla oblongata, 29S
nerves, 201, 284, 360

374 INDEX.

Air, atmospheric, composition of, 4, note

changes in, effected by respiration, 2, 4, 82, 86, 368
in lungs, residua), stationary and tidal, 94, 96, 368
odoriferous, 215
Air cavities in turbinal bones, 214
Air cells in lungs, 84, 96
Air tension in ear, regulation of, 240
Albumen (white of egg, albmti, white), in blood, 73

as a food, 144, 146
Alimentation {(do, I nourish) function of, 143 to 168

organs of, 150 ; &.C.
Alimentary canal, mucous lining of, 316

muscular fibres of, 174
Alkaline reaction of bile, 129
blood, 72
living muscle, 172
lymph, 72

pancreatic juice, 164
sweat, 120
Alveolus (a small hollow vessel), 153

growth of, 345
Amoebae, (a'lotoiiSb?, reciprocal) likeness of colourless corpuscles to, 65
Amoeboid movements of white corpuscles, 170, 326
Ampulla; {ampitlla, a flask or bottle) of semicircular canals of ear, 218
Amputation {anily. around ;puto, I cut) of tongue, effect of, 199
Amyloids (d/xvAoj', starch) as food, 144
digested in mouth, 166
not acted on directly by gastric juice, 159
Animal diet, result of, 74
'"Animal starch," 132

Anterior and posterior cornua (horns) of spinal cord, 282
Anterior nerve roots of cord motor in function, 283

connected with nerve cells of anterior cornua, 289
Anterior pyramids of medulla oblongata, decussation of, 298
Aorta (aet'pu), I take up or carry), 31
amount of pressure on, 368
abdominal, 106
valves of, 27, 38
Apex of heart felt in " beating" of the heart, 46

its position, 33
Appendix, vermiform, 162
Aqueous (a^?/«, water) humour of eye, 254
Arachnoid (o'pa'xiT)?, a spider or spider's web, eiSo?, shape) its fluid and

membrane, 279
Areolar (rt?rc7/r7, a little space), tissue, 9,323

Arteries (a'pnjp, that by which anything is suspended), bleeding in jets from,
when cut, 57
calibre of, regulated by vaso-motor system, 25, 52, 54, 138
elasticity of, 25, 45, 100
filling of, 45
pulsation of, 46
valves in primary, 27
walls of, 24
Arteries or Artery —

aorta, 27, 31, 38, 83

abdominal, 106
coronary, 31, 52
hepatic, 33, 126

INDEX. 375

Arteries or Artery —

iliac, io6

pulmotiarj', 27, 31, 83
renal, iii, 114, 116
splenic, 134
Articular (rtr>7/Vw///j, a joint) cartilages, 181, 321
Articulations of bones, 176 to 188

Arytenoid (dpvTfiva, a pitcher or ladle ; etSo?, shape) cartilages, 192
Asphyxia, (a, privative, a<j>v^oi I beat, of the pulse) modes of death from, T02
Association, law of, 302
Astragalus (a'a-Tpdya\o^, an ankle bone), 190
Atlas (d, euphonic, T\r<ixtii, I bear) vertebra, 183
Atmospheric (or/uc?, vapour ; <T(})aipa, a sphere) pressure, 100

how equalised in ear, 240
an obstacle to dislocation of hip, 186
opposed by elasticity of lungs, 89, 100
Auditory {audio, I hear) hairs, 219, 226
nerve, 216, 291, 296
sensorium, 233
spectra, 268
Auricles {auricula, a little ear) of heart, 35
Auricular appendage, 41
Auriculo-ventricular apertures, 36
Axis (afwr, an axle), cerebro-spinal, 6, 279

vertebra described, 1S3
Axis-fibre of Remak, 356
Azygos (a'^wyrs, unyoked) vein, 34

P..

Balance, physiological, how maintained, 4, iS
Ball and socket joints, iSi

capsular ligaments to, t86
Basilar {basis, a base) membrane of ear, 226
Beating of the heart, 46

Biceps muscle {bis, twice ; caput, a head), Its attachments, 176
Bicuspid {bis, twice ; cuspis, point of a weapon) teeth, 154, 347
Bile, secretion of, 125-131

flow of, into duodenum, 164
Bladder, 11 1

Blastomeres (jSAao-To?, a bud ; iJ.tpo<;, a division), 306
Blind spot of eye, 247
Blister, how formed, 309
Blood, 60-81

amount of lymph poured into, 73

arterial and venous, 77-81, 103

mixed in supply to liver, •'''

in capillaries, 23, 106

chemical composition of, 72

circulation of, 15, 50

evidence of indirect, 57

coagulation of, 6r, 68

corpuscles, 61-67, '35

crj'stals, 67, 80

functions of, 16, 74

gains and losses to, T07-110, 140-142

,76 INDEX.

Blcx)d, gases in, 73

glandular action on, 140
heat of, 17, S3, 72, 136
of hepatic vein, sugar in, 133
• microscopic appearance of, 60
oxygen carried by, 17, 73, 81
portal, 131 ^
specific gravity of, 72

of splenic vein, paucity of red corpuscles in, 135
transfusion of, 75
weight of, in body, 74, 365
Blood vessels, 22 ei S7ipra

peculiar epithelial lining of, 318
regulation of, by vaso-motor ner\-es, 54, 138, 29c
Blushing, how elTected, 53
Body, human, component parts of, 5, 365
diagrammatic section of, 7
elements present in, 366
Bone, canaliculi of, 332

cancellated structure of, 329
development of, 336
structure of, 174, 329
Bones, considered as levers, 176

number of, in body, 11
Bones, astragalus, 190
atlas, 183
axis, 183
clavicle, 11
coccyx, II
of ear, 228
femur, 175, 182, 336
humerus, 181, 183
hyoid, 151
ilium, II
incus, 229, 230
innominatum, 11
ischium, ir

of lower extremity, 11, 175, 177, 1S3
malleus, 227, 229
maxillary, 213
metacarpal, 182
nasal, 212

orbiculare, 230, note
patella, 11, 179
pelvic, 180
pubic, II, 180
radius, 176, 185
ribs, II, 87, 89, 93, 330
sacrum, 11
scapula, II

of skull, II, 131, 227, 339
temporal, 228

turbinal, 151, 212, 214, 264
ulna, 181, 184

of upper extremities, 11, 176, 183, i33
vertebrse, 6, 87, 279
Brain, base of, illustrated, 291

component parts of, 290 et supra

INDEX. 377

Brain, effect of destruction of, in frog, 55, 287, 299
respiration on, loi
venous blood on, 103

grey matter of, 294

hemispheres of, 293

injury to, death caused indirectly' by, 20

on one side affects opposite side of body, 25

lobes of, 293

localisation of powers in, 300

membranes of, 279

olfactory' lobes and ner\e form part of, 212, 362

optic nerve forms part of, 363

pia mater of, 279

portio dura of, 291

reflex action of, 301

sensation, mental action and will seated in, 14, 300

spinal cord continuous with, 6, 362

ventricles of, 292, 362
Bread, a mixed food, 148
Breathing, sec Respiration

Brewster, Sir David, quoted as to illusions, 209
Bronchi (/Spoyxo?, the windpipe), 83
Bronchial tubes, ciliated epithelium In, 89
Bninner, glands of, 163
Buccal (bucca, the mouth) glands, 152
Buffy coat of blood, 69
Bursae {bursa, a poueh), iSS

C^CUM {i.e. iiitestinuvi cerciim, the blind gut), 161
Calcic salts In bone, 10, 330, 338
Camera obscura (dark chamber) described, 253
the eyeball considered as, 257
Canal, alimentary, 143, 155-168

central, of spinal cord, 280
spinal, 6
Canaliculi of bones, 331

Canalis cochlearis (koxAo?, a spiral shell) of ear, 224
Canals, Haversian, 329

semicircular, of ear, 217, 227
Cancellous {cancelii, lattice-work) tissue of bone, 175, 329
Canine {cam's, a dog) teeth, 154, 346
Capillaries {capillns, a hair), continuous with veins and arteries, i;

dilatation of, under influence of heat, 138

exudation through, 16, 107, 118, 121

friction in, 45

heat evolved in, 136

lymphatic, 27

microscopic examination of, 23, 59

pulmonary-, oxidation of blood takes place In, 81
distribution of, 84

pulse lost In, 47

of stomach and intestines, 163, i65

structure of, 22

of the villi, 167

^valls of, 23

378 INDEX.

Capsule, Malpighian, 115, nj
Capsules in cartilafje, 321
Carbohydrates as food, 144

given up by the blood to the tissues, 107
Carbon {carlo, a coal), amount of, eliminated per diem, 87, 368
Carbonic acid, effect of, on blood corpuscles, 64

excess of, in venous blood, 2, 78, 103, T48
excretion of, by kidneys, 113

by lungs, 16, 102, 369
by muscle, 141, 149
by skin, 119, 369
mode of poisoning by, 104
a product of dissolution, 20
proportion of, in air, 86
Carbonic oxide gas, effect of, on blood corpuscles, 104
Cardiac (»cap5ia, the heart) dilatation of stomach, if 6

muscular tissue, 354
Cartilage (cartilago, gristle) on articulating surfaces, 176
growth and structure of, 319
in trachea and bronchi, 83
Cartilages, 10

articular, 11, 180
inter-articular, 181
sterno-costal, 319
thyroid, 191
Caruncula (dim. oicaro, flesh) lachrj'malis, 263
Casein {casens, cheese), 144

"Catherine wheel," continuous appearance of, on the retina, 248
Cells, ciliated, 89, 170, 214
rornification, of, 314
differentiation of, 306, 323
epidermic, 310

epithelial, continuous with epidermic, 9, 310
modified for sense organs, 205

of hearing, 216, 238
sight, 244
smell, 214
taste, 210
touch, 207
fat, 328

incessant reproduction of, 18, 120
liver or hepatic, 128
as living organisms, 65, 323, 353
nerve, 294, 359
nucleated, bone forming, 337
in capillaries, 22
of cartilage, 320
of connective tissue, 324
in embr^'onic tissues, 305, 322, 327, 353
pigment, 247, 255
secreting, 118, 141, 156
various forms of, 318
wandering, 326
Cement of teeth, 344
Centres, cerebro-spinal, 14, 27S
of ossification, 335
respiratory, 98, 103
vaso-motor, 290 *

INDEX. 379

Cerebellum (dim. oi cfri-!>?-inti), position of, 292

C'erebral (ccrel>r-unt, the brain) hemispheres, functions of, 299

Cerebro-spinal axis, 6, 278, 279

Check ligaments, 186

Cholesterine, (xoArj, bile ; (ntap, fat), 129, 131

Chondrine {\6v&po<;, cartiliage), 144, 319

Chordae tendinae, 38, 43

Choroid (^opioc, investing membrane of foetus ; ei5o9, form) coat, 247

pigment cells from, 255
Chyle (xvAo?, juice), formation of, 166

in the lymphatics, 24
receptacle of, 29
Chyme (\ujab<r, pulpy juice) 161, 165
Cilia {ciliuDt, an eyelash) described, 170
on bronchial epithelium, 89
on part of nasal mucous membrane, 214
from the windpipe, length of, 370
Ciliary ligament and muscle, 256
processes of choroid, 255
Circulation of the blood, 15, 22

control of, by the vaso-motor system. 25, 54, 290
constant of, 368
course of, 30

effects of respiration on the, 99
evidence of the indirect, 57
in kidney, 117
Circulation, portal, 51, 131
Circumduction (a leading around), 187

Circumvallate {circjun, around ; valhnn, .a wall) papillae, 209
Cistern of the chyle, 29
Clavicle {clavicnln, a small key), ri
Clot of blood, 68

Coagulation {con, together ; ago, I drive) of blood, 61, 63
Coal-gas, risk from breathing, 105
Coats of arteries and veins, 24
Coccyx (kokkv^, a cuckoo), 11
Cochlea (xoxXta?, a spiral shell) of ear described, 222-227

functions of, 235
Cochlear nerve, 236
Cold, respiration affected by, 102

sensation of, 208
Collaginous (ic6A.Aa. glue ; yevvdu). I produce) fibres of connective tissue, 324
Colon ((ctjAo;', a part or division), 163
Colours, complementary, 248
Colour-blindness, 249
Colourless corpuscles of the blood, 61

changing form of, 64, 170, 326
possibly produced in spleen. 135
relative number of, 72
size of, 369

typical nucleated cells, 305
Columnse carnese (fleshy columns), 38
Combination of muscular actions, 13, 288, 299, 301
Commis.sural (con, together ; mitto, I send) cords, 290, 303
Complementary' colours seen as result of retinal fatigue, 24S
Concha (/cdyxo?, a sea shell) of ear described, 232
Concussion of brain, 13
Conduction of impulses, 289

38o INDEX.

Cones (ntdji/o?, a fir cone) of retina, 244, 257
abundant in yellow spot. 246
width of, 370
Conjunctiva (fiJ«, together ; 7««^(7. I join), 263
Connective {con, together ; necfo, I fasten) tissue, 9, 323

corpuscles, 322, 324
fibres of, 324
fibrils of, 370

perimysium formed of, 348
ossification of, in skull development, 340
varieties of, 326
Consciousness, states of, 202

Consonants {con, with ; sono. I sound), pronunciation of, 198
Constants, anatomical and physiological, 365
Contact {con, with ; tango, I touch), sense of, 203
Contractility (c^«, together; trnfw, I draw) of bronchial tubes, 89
of colourless corpuscles, 64, 353
of muscular fibre, 171, 353
Contraction of heart dependent on its ganglia, 53
rhythmical, 41
of hollow muscles, 173
of intercostal muscles, 90
of iris, 174, 25s
of muscles, 10, 14, 25, 41, 283
of muscular coat of arteries, 25, 53
of muscular fibre, 171, 201, 284
peristaltic, of gland ducts, 174

of intestines, 164. 174
of sphincter muscles, 112, 164
Convolutions of brain, 294
Cord, spinal, 6

described, 279

combined muscular actions directed by, 14, 28S
course of impulses along. 289
vaso-motor centres in, 290
Q,orr\&2^{comeits, homy), 253
Comified cells, 314
Comu (a horn) of spinal cord, anterior and posterior, 282

from lateral ventricle of brain, 294
Coronary {corona, a crown) arteries, 31, 53
Corpora albicantia, position of, 252

quadrigemina. 292, 295
Corpus callosum (the hard body), 292, 294

striatum (the striped body), 294
Corpuscles {corpuscuhim, dim. o{ corpus, a body) of the blood, 6i
effect of the spleen on. 135
measurement of, 314, 369
of connective tissue, 324, 326
of the spleen, 134
tactile, 207
Corti, organ and rods of, 226, 238
Coughing, 94

Cranial nerves, arrangement of, 295
Crassamentum (dregs), 68

Cribriform {cribra, a. sieve \ forma, shape) plate, 213, 362
Cricoid («cpi'(to?, a ring) muscle, 191
Cricoarytenoid muscle, 194
Crico-thyroid muscle, 194

INDEX. 381

Crista acustica (acoustic cre-%t) 218

Crossing over of nenous impulses in cord, 289, 298

in medulla oblongata, 298
Crown of tooth, 341
Cnjcial ligament, r86
Crura cerebri, 292
Crystals in blood, 67

change of colour of, by oxygenation, 80
doubly refracting, 272
Crystalline lens, 255

Cubic feet of air needed for respiration, 105, 369
Curvatures of stomach, 156
Cutaneous excretions, constant of, 369

Dav and night, var>4ng amount of oxygen absorbed in, 102
Death from asphyxia, 102

of the blood, 66

general and local, 18

immediate causes of, 19

of muscle, changes caused by, 172, 350

stiffening after, 172
Deciduous teeth, 346

Decomposable animal matter given off by lungs, 86
Decomposition after death, 20
Decussation of the anterior p%Tamids, 298
Delirium tremens (trembling delirium), 269
Delusions of the judgment, 268, 271

optical, 270
Dental {dens, tooth) pulp, 342

tissues, 328, 341
Dentine, 342
Dentition, 346
"Derbyshire neck," 134
Derma (Sep/xa, skin), 8, 308
Dextrine {dexter, right-handed, from the direction of light polarised throiigl

. ")' ^45
Diabetes, a form of, produced by injur>' to the medulla oblongata, 297
Diaphragm (6ia, across ; <}>pd<T<T<ti, I separate by a fence)
action of, in respiration, 91
connection of pericardium with, 34
of camera obscura, 257
Diaphysis (fiia, across ; (f)vui, I grow) of rudimentary bone, 336
Diastole (5i', apart : crTeAAaj, I place). 42
Diet, amount of oxygen absorbed depends on, 102

best form of. 147
Differentiation of cells, 306, 323
Diffusion of gases. 78

Digastric (5t for SI?, twice ; yaarrjp, the belly), muscles, 189
Digestion, artificial, 158

constant of, 367
secondary', 131
Digits of hands and feet, 5
Dim bonds of striated muscular fibre, 350

382 INDEX.

Division of labour in cells, 307

nucleus of epidermic cells, 311
mammalian ovum, 306
Double hinge-joint, i3i

vision, as result of squinting. 275
Drill, reflex nature of actions taught by, 302
Drinking, mechanism of, 155
1 >rum of the ear, 228
Duct, bile, 128, 135
hepatic, 126
lachrymal, 264
pancreatic, 135, 157
thoracic. 28
Ductless glands, 134
Duodenum {duodeni, twelve, from being twelve fmger-breadths in length), 161

secretions flowing into the, 164
Dura mater, 279
Dyspnoea (Sus, bad ; Trveco, I breathe), 105

Eak described, 215-240

experiment on blood supply to, 53, 290
Education, basis of the possibility of, 302
Efferent {ex, out oi\/ero, I bear), impulses, course, of. 289
nerves defined, 284

muscular fibre contracts by means of, 201
Elasticity of artery walls, 25, 45, 48
cartilage, 319
lungs, 89, 99
muscle, 172
Elbow joint, 181, 183
Electrical fishes, efferent nerves of, 285
Elements present in human body, 366
Embryo, growth of bones in, 335, 339

connective tissue in, 327
muscle in, 353
teeth in, 344
red corpuscles nucleated in, 66
Embryonic form of all tissues. 305
Emotions, effect of, on the he.irt, 55

on perspiration, 123
on the vaso-motor system, 53
painful, tears a consequence of, 264
Emulslfication of fats, 165
Enamel of teeth, 154, 344

organ, 345
End-bulb of nerve fibre, 207
End-organs of special sensations, 235, 238, 300
Endocardium (cVSoi', within ; Kap8i.a, the heart), 36
Endolymph {evSov, within ; lyutpJui, water) contained in ear-sac, 21^

vibrations of, 235
Energy (Iv. in ; cpyov, work) supplied by oxidation, 5, 17
Epidermis (sTri. upon ; 6e'p/u.a, skni), 8

breadth of superficial cells of, 370
cells of, converted into horn, 317

INDEX. 383

Epidermis, composition of, 308. 311

continuous with epithelium, 9, 310
an excretory organ, 314
growth of, 311
non-vascular. 22
its relation to the derma, 313
scales of, continually shed, 120, 309
Epiglottis (eJTi, upon ; y\wTTa, a tongue), 82, 152
Epiphyses of rudimentary' bone, 336
Epithelium (en-i. upon ", OdWiHy I grow)
auditorj", 216, 226, 235, 238
cells of, incessantly reproduced, i3

nucleated, 310, 31S
ciliated, 170

in bronchial tubes, 89
in nasal mucous membrane, 214
epidermis, continued into, 9
modified in sense-organs, 205, 207, 214
non-vascular, 22, 318
of serous cavities, 318
secreting, in sweat glands, 12

in tubules of kidney, 117, 115
Epithelial tissue, 308
Erect position, how maintained, 12 "
Ether, vibrations of, physical basis of light, 246
Eustachian tube, 152. 229

probable office of. 24a
Evaporation from the lungs, 87
from the skin, 137
Excretions (^.r, from ; cerno. I separate)

amount of oxygen contained in, 3, 143
solid matter in, 143, 369
Excretory organs, 16, 106
Expiration and inspiration {exspiro, I breathe out). 85. 101

usually performed silently, 194
Expired air, analysis of, 86, 368
Extension of limbs, 1S7
Eye, the, 241-264

accommodation of, 258
blind spot of, 247
muscles of i38, 261
nerve supply to, 293
yellow spot of, 245
Eyeball, component parts of, 253
Eyelids and eyelashes, 262

Face, cavity of, 8

Facial nerves, 296

Faeces {fcex, grounds). 15, 150. 168, 367

Fainting effected by action of the pneumogastric, 55, 57

Faintness, sense of, 203

Fangs of teeth. 341

Fascia (a band) of a muscle, 348

Fat cells, 328

Fatigue, a cause of, 102, 203

384

INDEX.

Fatigue of retina, 2^3

Fats, absorbed by the lymphatics, 167

emulsified in duodenum, 165

as food, 144

given up from the blood to the tissues. 107

not acted on directly by^ gastric juice, 159

not sufficient alone to support life, 146
Fatty tissue, 327
Fauces. 152

Femur (the thigh), structure of, 175
Fenestra (a window or opening) ovalis, 227

rotunda, 224
Ferments in blood, 72

in caecum, 167
Fibres of connective tissue, 324

muscular, 171, 369 ; breadth of, 369

nervous, 172, 369
Fibrils of connective tissue, 324

breadth of, 370
muscle, 349, 352
Fibrin, 67, 70
Fibrinogen, 71
Fibrous tissue, 9

arteries sheathed by, 24
Figures, Purkinje's, 250

Filiform (Jilium, a thread \ forma, a shape) papillae of tongue, 209
Fishes, electrical, efferent ner\-es of, 285
Fissure of Sylvius, 292, 294
Fissures of spinal cord, 279
Flexion of limbs, 187
Fluid, arachnoid. 279

of labjTinth of ear, 216

of pericardial sac, 34, 71
Food, average amount taJcen. 143. 367

effect of, on respiration. 102

necessary constituents of, 3, 144

oxidation of. in the body, 5, 17, 149

taken up by the blood, loS
Food-stuffs classified, 144, 150
Foot, the, II

as lever, 177
Foot-tons, work of heart estimated in, 36B
Foramen (a hole ; {rom/oro, I pierce), nutritive, of bone, 329
Foramina. inter\ertebral. 280
Friction of blood in capillaries. 45, 48
Frog, experiment on, as to action of pneumogastric, 55
refle.v actions, 287. 299

rate of transmission of nervous impulse in, 369
Frontal and parietal bones, ossification of, 340
Fulcrum, relative position, of, in various levers, 177
Fungiform papillae of tongue, 209

Gall-bladder, 126, 129

storage of bile in the, 164
Galvanism, effect of, on spinal cord and ner\es, 14, 283
Ganglia (yayyAioi', a hard gathering) of the heart, 55

INDEX. . 385

Ganglia, lymphatic, 27

on sensory roots of fifth pair of ner\'es, 2c,6
sympathetic, 6, 278, 303
Ganglion of the posterior root, 280
Gastric {yaarfip. the stomach) glands, 156

juice. 157, 159
Gases, diffusion of, 78

poisonous, 104

proportions of, in atmospheric air, 4 ; noit
Gasping, how caused, 99
Gelatine {gelo, I freeze), 144

obtained from connective tissue, 323
General death precedes local death, 19
Germinal spot and vesicle, 306
Glands {glans, an acorn), a source of loss to the blood, 140

structure of, 139
Glands of Brunner, 163

buccal, 152

cutaneous, 314

ductless, 134

gastric, 156

lachrymal, 263

of Lieberkiihn, 140, 163

lymphatic, 27, 134

mesenteric, 29

parotid, 152

racemose, 140

salivary', 109

sebaceous, 120, 140, 314

sublingual, 152

sub-maxillary, 152
Glasses, multiplying, 272
Globulin, 71

Glomerulus {dim. oi glomus, a clue of thread) of kidney, 117
Glottis (^\uma, the tongue) described, 191

position of, 152, 211

under control of the medulla oblongata, 296
Glosso-pharyngeal nerve, 209. 291

both motor and sensory in function, 296
Gluten {s:luo, I draw together), 144

in bread, 148
Glycocholic (7A.UKVS, sweet ; \ok-ii, bile) acid, 129
Glycogen (■yXv/cu?, sweet ; ■yti'i/aa), I produce) in liver cells, 125
conversion of, into grape sugar, 132
non-nitrogenous, 173
Goitre {guttur, the throat), 134
Granular layers of eye, 244
Grape-sugar formed from glycogen, 132
Grey matter of brain, special nature of, 294
in medalla oblongata, 294
of spinal cord, 282, 289
Gristle, 10
Gullet. 152

passage of fluids in. 155
Gum, of mouth, 153, 341
Gums as food, 145

Gustatory {^sto, I taste) nerve, 209, 296
Gyri of brain surface, 294

C C

386 INDEX.

H.CMATix (aifxaTivo?. charged with blood), 63
Haemoglobin (al/xi, blood ; globus, a globe), 63
acted on bj' carbonic acid, 104
combination of, with oxygen, 73, 80
cr5"stallisation of, 67
Hair, non-vascular. 22

its growth limited, 315
Hair-like processes on auditory epithelium, 216.. 219, 220, 239
Hairs, growth of, 314-317

measurement of, 313
roots of. 121
Haversian canals, 329
Hearing, mechanism of, 215-240
Heart, action of, helped by respiration. loi

increased by irritation of sympathetic, 303
stopped by irritation of pneumogastric, 55, 297, 303
di\^ionsof, 35
ganglia of. 55

muscular fibres of, 36, 173. 354
rhythmical contraction of, 15, 42, 55
size of the, 33
.sounds of the, 46
work done by the, 368
Heat, constant loss of in the body, 3, 107, 135
produced by oxidation, 17, 108, 136, 149
regulation of, 137
sensation of, 208
Hemispheres of brain described, 294
Hepatic (jj^ap, the liver) arterj-, 33, 126
cells, 128

their action, 131
duct, 126
vein, 127
Herbivorous animals, development of caecum in, 161, tiole, 167
Hilusof the kidney, in
Hinge joints, 181

Hip-joint, section of. 182 _

Histology' (larb?, a tissue ; A070?, a discourse), defined. 304
Histological measurements, 369
Hollow muscles, 173

Homoiomera (omoio?, like ; ntpo^. a diWsion), 304
Hoops, cartilaginous, of trachea. 83
Horn, epidermic cells converted into, 317
Humerus (the shoulder) articulation of, 184
Humours of the eye, 254

Hydrochloric (vfiwp, water ; x^f^po^' pale green) acid in gastric juice, 157
Hydrogen, (vSwp, water ; ytwdoi. I produce) in foods, 145

sulphuretted, poisonous etTects of. 104
Hyoid (v, the letter upsilon ; elfios, shape) bone, 191
H)-poglossal (v-b, beneath ; y\<ZTTa, the tongue) ner\'e, 291

lLEO-CyCC.\L valves, i6t
Ileum (elkeoj, I roll), 162
Iliac {t7ia, the flanks) arteries, 106

INDEX. ' 387

Ilium, II

Illusions, spectral, 269
Imperfect joints, 180

Impression, retinal, corrected by sense of touch, 271
Impulses, riervous, conduction of, 289
decussation of, 298

require time for propagation, 285, 369
Incisor (iHci do. I cut) teeth, 154, 346
Incus (an anvil). 230

Injurj' to medulla oblongata, result of, 297
spinal cord, result of, 13, 283, 285
Innervation, 278

Innominatum (nameless) bone, 11
Insensible perspiration, 119
Insertion of a muscle, 188
Inspiration (in, spiro, I breathe)

heart's action helped by, loi
mechanism of, 90
rate of, per minute, 84, 97, 368
Integument (/«, upon ; tego, I cover) double, 8, 309
Intelligence destroyed by removal of cerebral hemispheres. 299
Inter-articular cartilages, 181
Intercellular substance of cartilage, 320
Intercostal {inter, between ; casta, a rib) muscles, 89

nerves, 98
Intestines, all food-stuffs dissolved in, 166

small and large, 161
Intralobular vein, 127

Inverted position of retinal image, no obstacle to uprigtit vision, 271
Iris (a rainbow) described, 255

muscular fibres of. 174
Irritation of cut end of sjmpathetic, 54, 303
motor nerves, 284
pneumogastric. 303
trunk of spinal nerve, 282
upper dorsal region of cord, 290
Ischium (i(7xioi', the hip), n

Jaw, lower and upper, 153, 154

development of teeth in, 344
Jerks, blood issues from cut artery by. 47, 57

obviated by elasticity of tubes, 48
Joints, ball and socket, i8r

exemplii"ying lever action, 178
hinge, 181

perfect and imperfect, 180
pivot, 183
Judgment combined with sensations, 266
delusions of the, 268-271
visual images interpreted by the, 274
Juice, gastric, 157

intestinal, 163
pancreatic, 164
Jumping, 190

C C 2

388

INDEX.

Kidneys, amount of excretion from, 369

described, iii

excretory functions of, 16, 113

minute structure of, 114

position of, 6
Kreatin (Kpia^. flesh), 173

Labyrinth (A.ojSv/jti'dos, a maze) of ear, membranous, 217

osseous, 219
Lachrj-mal (/ackrjma, a tear) duct and sac, 264

gland, 263
Lacteal (iac, milk) radicles and vessels, 163

absorption of fa., by, 166
Lacteals, 29, 163
Lactic acid, 173
Lacunae of bones, 331
Lamina spiralis (spiral plate) of ear, 224
Larj-nx {Kdpvy^, throat), 191
artificial, 199
voice produced by, 190
Leather made from the derma, 9
Lens (a lentil seed), adjustment of, 259

crj'stalline, 251, 255
Lenses, concave and convex, 273
Levers (/evo, I raise), bones considered as, 10, 176

three kinds of, 177
Lieberkiihn, glands of, 140, 163
Life accompanied by oxidative changes, 136
depends on circulation and respiration, 20
individual, of cells, 65, 323, 353
as physiological work. 2
Ligaments {/i^^o, I bind), 181, 186

forming pulleys, 188, 262
suspensory, of lens, 255
vocal, 191
Ligamentum nuchae, 327
Light, sensation of, in the sensorlum, 246
Limbs, 5

Lime, salts of, in bone, 10, 330, 338
Lime-water, how changed by breathing through, 2
Liver, blood supply to the, 128
described, 125
glycogen stored in the, 132
secretion of bile by the, 129, 164
vessels of the, 33
Lobes of the brain, 293
Lobules of the liver, 127
Local death unceasing, 18

Locomotion {locus, a place ; utoveo, I move) how effected, iS
Long sight, 261

Losses of the blood, 107, 119, 131, 140
body, 366

INDEX. 389

Luminous impression on eye, duration of, 247
Lungs, absorption of oxygen by, 17, 81, 36^

elasticity of, 89, 99

as excretory organs, 16, 87

position of, 6

structure of, 84

veins and arteries of, 31
Lymph ijympha, water), 24, 75
Lymphatic system and glands, 24, 27, 134

Macula acustica (acoustic spot), 219

lutea (yellow) of retina, 243, 245
Madder, experiment with, as to growth of bone a, 334
Malleus (a hammer), 227, 229
Malpighian capsule, 115, 117
Malpighii rete, 121, 309
Mammal, embryonic growth of a, 306
Manufacture of bile acids in liver, 131

of some constituents of urine in kidney, 119
of glycogen by hepatic cells, 133
Marrow in bones, 174

formation of, 338
Mastication, 155
Matter, its changes, 20

solid, lost by perspiration, 124

passed from alimentary- canal, 143
kidneys and skin, 369
Maxillary {tnaxilla, jaw-bone), bones, 213
Measurements, liistological, 313
Meat " boiled to rags," 348
Meatus {tftco, I pass) of ear, 228

Medulla oblongata (oblong marrow), arrangement of grey and white matter
in, 204
decussation of impulses in, 298
effect of venous blood on, 103
injury- to, result of, 19, 297, 298
nervous centre for respiration in, 97, 98, 103, 297
for vaso-motor ner\'es. 290, 297
Medullary- cavity of bones, 329
matter of hairs, 317
substance of the kidney, 114
Medullated nerve fibres, 360
Meibomian glands, 263
Membrane, arachnoid, 279

limiting, of eye, 244
mucous, 9

permeability of, 120, 159
of Reissner, 226
"serous," 34, note
vibration of, 231
Membranous labjTinth of ear, 217
Mesentery (jiecroi, middle ; eprepov, intestine/, 29
Metacarpal (jj-erd, beyond ; /capTrbs, the wrist) bone of thumb, 182
Migratory cells, 326

3P0

INDEX.

Milk teeth, 345

Mind not the sole governor of muscle, 13
Minerals as food, 144, 145, 366
Molar (w^A», 1 grind) teeth, 154, 346

Molecular {moiecnla. dim. of tiio/cs, a mass) change in cerebral substance,

300
in stimulated nerv'eSj
202, 216
vibrations, 233
Mortification (>/wrs, death \ facto, I make), 19
Motion in living body incessant, i, 170
Motor fibre, 172

ner\'eSj 201, 284 _

composition of, 355
plates, 358
Motores oculi ner^'es, 295
Mouth, 150 _

epithelial scales from interior of, 311
Movements, amoeboid, 65, 170, 326
ciliary, 170
of joints, 176-189
Mucous membrane. 9

of alimentary canal, 318
olfactory, 214
Mucus, 9

Murmurs, respirator^-, 99

Muscle (inuscuhis, a little mouse), contractility of, 10 25, 41, 171, 201
corpuscles, 350, 353
as organ and as tissue, 347
striated, 36, 171, 172. 347
unstriated, 24, 171, 255, 354
waste in contraction of, 149
Muscles, attached to definite levers, 174
carbonic acid secreted by, 141
change in, after death, 172, 350
composition of, 171, 173
death of, 19

changes caused by, 350
hollow, 173

insertion and origin of, iSS
oxidation of, 17, loS, 149
Muscles, arj'tenoid, 194
biceps, 10, 1S8
ciliarj-, 256, 260
crico-ar^-tenoid, 194
digastric, 189
facial, 296

intercostal, external and internal, 83, 90
oblique, of the eye, inferior, 262

superior, i33, 262, 295
papillarj', 38, 43
pharyngeal, 296
rectus, of abdomen, 179

of eye, external and internal, 262, 296
superior and inferior, 261, 295
of leg, 178, note
stapedius, 231, 239
tensor tjnnpani, 231, 239

INDEX.

Muscles, thyro-arj'tenoid, 195

triceps, 188
Muscular coat of arteries, 24

fibre, breadth of, 369

fibres of the heart, 36, 174,355

radiating, of iris, 255
sense, the, 203
tissue, development of, 353
Musical sounds, how produced, 236

notes, varjing with the tension of vocal chords, igt
.^^yelin, 358
Myosin (mO?, a mouse), 144

coagulation of, in rigor mortis, 173

Nails, growth of, 314

non-vascular, 22
Nares (nostrils), anterior and posterior, 211
Xasal {nasus, nose) bones, 212

cavities, ciliated cells in, 171
" Near sight," 260
Ner\'es, afferent or sensory, 201, 283, 360

arterial, 25

auditory, 219, 238, 291, 296

cochlear, 236, 238

cranial, 295

effect of irritation on, 118, 141, 246, 283, 286

efferent or motor, 201, 283, 355

facial, 296

glosso-pharj-ngeal, 209, 291

gustatory-, 209, 296

of the heart, 55

hypoglossal, 291

intercostal, 98

motores oculi, 295

olfactory, 211, 214, 291, 362

optic, 246, 297, 363

phrenic, 98

pneumogastric, or vagus, 55, 291, 297

posterior and anterior roots of, 280

renal, 118

of special sensations, end-organs of, 235

spinal, 280, 360

spinal accessory, 291, 296

sweat, 124

s3-mpathetlc, 6, 53, 278, 303

trigeminal, 291, 296

vaso-motor, 25, 52, 54, 138, 290

vestibular, 236
Ner\'e-cells of cord, 359

breadth of, 370
in olfactory- "nerve," 362
absent from optic "ner\'e," 36
in grey matter, 282

391

392 INDEX.

Ner\'e-cells in nene centres, 279

of sympathetic ganglia, 361
Nerve centre, spinal cord an independent, 288
Ner\'e centres, composition of, 279
function of, 14, 235
Nerve-fibres, in blind spot of eye, 250
diameter of, 356, 370
in ear, 235
medullated, 360
nodes of, 356
nucleated, 356
structure of, 357
in tactile corpuscles, 207

white matter of cord and brain composed of. 282, 294
Ner\'e tissue described, 355
Ners'e roots, functions of, 283
Nervous apparatus, duplexity of, 278
impulse, conduction of, 289

rate of, 369
molecular change in ner^•e-fibres caused by, 141, 171, 205,

2351.246
transmitted from brain by spinal cord, 288
sj-stem, 278

as combining organ, 18
as controlling circulation, 25, 53
evaporation, 137
glandular action, 123, 141
muscular action, 283
respiration, 97, 103
Neuraxis, 356
Neurilemma (yevpop, a ner\-e ", Xefjifj-a, a peel or skin), 207, 356, tw/e

continuous with sa.co^eiuma, 358
Nitrogen (virpov, potash ; yevva'tu, I produce) not absorbed by lungs, 86
in proteid foods, 144, 147
starvation from lack of, 146
in urea, 113
Nitrogenous waste, excretion of, 113, i.;6
Nodes of nerve-fibres, 356
Non-medullated ners'e-fibres, 361
Non-vascular tissues, 22
Nose, 211

Nucleated cells, bone-forming, 337
in capillaries, 22
in cartilage, 320

of epidermis and epithelium. 310. 318
in lacunae of bone, 333

all tissues primitively composed of, 305, 353
Nucleolus of ners-e cell, 360

ovum, 306
Nucleus (a kernel) in white corpuscles, 65
division of, in growth of o\-um, 306
in cells of capillary- walls, 22
in nerve-fibres, 356
in unstriped muscular fibre-cells, 354
Nutrition effected by circulation of blood, 16
Nutritive foramen of bone, 329

value of food not solely measured by chemical analysis, 148, ftnfe

INDEX. 393

Oblique muscles of the eye, i88, 261, 295
Ocular spectra, 269

Odontoid (66ous, oSoiro?, a tooth ; et5o?, form) process, 163
Odontoplasts (65ovj, a tooth ; nXdaa-to, I form), 345
CEsophagus (oicro), obsolete=(^e'pai, I bear ; (}>ayelu, to eat), 83, 152
Olecranon ((oKevr), the elbow ; /cpaVo?, a helmet), 18 1
Olfactory (plfacio, I smell) lobes, 213
membrane, 214
nerves, 295

not traceable to medulla oblongata, 297
prolongations of cerebral hemispheres, 297, 362
Optic nerve, 241, 291, 295

not directly excited by light, 246
a prolongation of third ventricle of brain, 297, 363
ramifications of, 244
thalami, 292

grey matter in, 294
Optical delusions, 270
Ora serrata (serrated border), 257
Orbicular (prbiculus, a small round ball) bone, 230
Orbicularis muscle, 241, 261
Organ of Corti, 226
Organules of special sense, 205, 359
Origin of a muscle, 188
Osmosis (wcTjab?, impulsion), 159

of peptones, &c., into the villi, 167
Osseous labyrinth of ear, 219
tissues, 328

origin of, 335
Ossicles {ossicula, a little bone) auditory, 229, 233
Ossification, centres of. 335

Osteoplasts (^(niov, a bone ; nkdavto, I form), 337
Otoliths (ov?, torb?, an ear ; Ai'^os, a stone), 221
" Outness," sense of, accompanying sense of sight, 251

of smell, 266
Oven, heated, conditions of safely remaining in, 140
Overtones, their nature, 237
Ovum, mammalian, described, 306
Oxidation, change to arterial blood caused by, 81
of proteid matter, 146
in tissues, the source of energy, 5

of heat, 17, 108, 136, 149
Oxygen (o^u?, acid ; yivvdu), I produce), absorption of, by the lungs, 17, 86,
102, 108, 133
amount of, consumed, 368

blood corpuscles apparently flattened by presence of, 64, 80
colour of arterial blood caused by, 80
combination of, with haemoglobin, 73, 80
effect of privation of, 104
excess of, in arterial blood, 78
in excretions, 4, 14B

Palate, hard, 150

soft, 152, 211
Palpitation caused by emotions, 55

394 INDEX.

Pancreas (irai/, all ; Kpeaq, flesh). 140

poMtion of. 161
Pancreatic juice, 164
Papilla, dental, 345

ofhair, 316, 317
Papillae, tactile, 206

of tongue, 209
Papillarj' muscles, 38, 43
Par vagum, or pneumogastric ner\es. 296
Paraglobulin, 71
Paralysis (-apa, beside ; At'eo, I loosen), a result of division of spinal cord, 285

injur}- to brain, 298
P.'.rotid (TTOfia, beside ; oJs, (Jto?. the ear) gland, 152
Patella (a dish or plate), 11. 179
Pelvis (a basin), 11. i3o
of the kidney. 114
Pepsin (jren-Tw, I digest). 157, 159
Peptone, 158

how formed. 165. 166
solubility of, 159
Perfect joints, 180
Pericardium {-ep\. about ; KapSia, the heart), 33

contents of, 71
Perichondrium (jrepl, about ; x6vSpo<;. cartilage), 319
Perilymph (irepl. about ; lyinpha, water), ear-sac surrounded b}-, 216
Perimysium {j^epi. about ; ^^.v%, a muscle), consists of connecti\'€ tissue, 348

continuous with i>erineurium. 35S
Perineurium (jrepl, about ; vevpov, a ner\e). 355

continuous with pia mater of cord, 359
Periosteal bone. 537
Periosteum ("epl, about ; hr-eov. a bone), 329

development of. from perichondrium. 336
Peritoneum ("cpi. about; reiVo), I stretch) described. 112
intestines and stomach enveloped in, 161
liver surrounded by, 125
Permeability of membrane. 120. 159
Perspective, aerial and solid. 270

Perspiration (per. through ; sp/ro, I breathe) affected by emotion, 123
amount of matter lost by. 124. 369
sensible and insensible, 119
Petrosal (irerpa. a rock) bone. 215
Phalanges ((}>dXay^, a rank of soldiers), 5
Pharynx {4>dpvy^, the throat), S2, 152
Phosphates excreted by kidney, 113
Phosphene (<^c>Js, light ; (^aiVco, I display). 249
Phosphorus sometimes present in proteids, 14

present in human body, 366
Phrenic (<^pi|i', the diaphragm) ner\es, 98
Physiology, human, defined. 2

ultimate analysis of, 304
Pia mater, 279. 362
Pigment (pigvientian , paint) cells of choroid, 246, 255

of web of frog, 56, 58
Pillars of the diaphragm, 91

of the fauces, 1 52
Pineal body. 292

Pituitarj- (pituita, phlegm or mucus) body, 292, 293
Pivot joint, 183

INDEX.

395

Plasma (n\d(Tiia, workmansliip) of the blood, 61-63

f'il)rinoj:;en in, 71
Pleura (wAeupa, a rib or side), 87
Plexuses of tbe sympathetic system, 303

Pneumogastric {nfevfj-wf, lung ; yaa-Tr'ip, the stomach) nerves, 55, 296
heart's action arrested by means of, 57, 297, 303
respiration affected by, 99
Poisoning by carbonic acid, 103

by sulphuretted hydrogen and carbonic oxide, 104
Pons Varolii, 292
Fortal (^orin, a gate) circulation, 51, 131

passage of peptones into the, 167
Portio dura of brain, 291

and portio mollis of '' 7th pair " of nerves, 296
Position, erect, how maintained, 12
Posterior cornu, 282

nerve roots, sensory in function, 283
root, ganglion of the, 280
Pressure, atmospheric, 100

on heart, diminished during inspiration, 99
ee]ualised in ear, 240
sense of, 203
" Primitive sheath " of nerve-fibres, 356
Pronation (prufius, face downwards) of limbs, 184

Proteid (rrptu-o?, first ; el5o9, shape) material acted on by pancreatic fluid, 156

blood corpuscles formed of, 63
dissolved by gastric juice, 158
as food, 3, 144, 146, 367
given up to the tissues from the

blood, 107
nitrogen supplied by, 146
Protoplasm, colourless corpuscles formed of, 65, 305

of ovum, 306
Pseudoscope (i^ev5>j?, false ; (TKoneta, I view) action of, 276
Psychical (v//vx>). the spirit) phenomena, connection inconceivable between

molecular changes and, 301
Ptyalin (tttvoj, 1 spit ; aA.ii'09, salted), properties of, 153, 156
Pulleys, ligamentous, 188, 262
Pulmonary {pitlfno. lung) capillaries, 81
Pulp cavity of tooth, 342
Pulse, the, 46

lost in capillaries, 47
venous, loi
Punctum lachrymale (lachrymal point), 263
Purkinje's figures, how produced, 250
Pylorus (TTvAwpb?, agate-keeper), 156

of the kidney, 114
Pyramids, anterior, of medulla oblongata, 298

QuADRiGEMlNA, corpora, 292, 295

Rabbit, experiment on ear of, 53

Racemose {racetims, a bunch of grapes) glands, 140

Radiating muscular fibres of iris, 255

396 INDEX.

Radicles, lacteal, 163

Radius (a ray or spoke of a wheel). 176

articulation of, 185
Recti (straight) muscles of the eye, 261

ner\-e supply to, 296
Rectum (intestinum rectum). 163
Rectus muscle of abdomen, 170

of leg, 178
Receptacle of the chyle, 29
Red corpuscles, 61

action of oxygen on, 64, 80
possibly broken up in spleen, 135
size of, 62, 369
structure of, 63
Reflex action, 202

of the brain, 301
of the cord. 287. 299
in coughing, 97
Relssner, membrane of, 226
Remak, axis-fibre of, 356
Renal {ren, a kidney) arter.-. 15
excretion, 113

constant of, 369
Reproduction of tissue, 19, 335
Residual air, 94, 368
Resistance to effort, sense of. 203
Respiration, 77-105

constant of, 368
costal. 93

cubic feet of air needed for, 105, 369
diaphragmitic, 93
effect of, on circulation, 99
essential of, 77
mechanism of, 87-97, 179
ner\-ous apparatus of, 97
rate of, per minute, 84, 368
Respiratory centre in medulla oblongata, 98, 103, 297

sounds, 99
Restlessness, sensation of, 203
Rete (a net) Malpighii, 121, 309
Retina {rete, a net) described. 241, 309

distinguished from fibres of the optic ner^'e, 250
its sensibility soon exhausted, 248
Retinal impressions corrected by sense of touch, 271
Rhythmical (pvOfjLO^, measured motion) pulsation of heart, 15, 41, 42, 53
Ribs. 11. 89, 330

Rigor tnortis (stiffness of death), 172
Rods and cones, layer of, 242, 244

affected by light, 250
Rods of Corti, 226. 238
Rod-shaped cells of olfactory nerves, 214
Roots of spinal ners'es, anterior and posterior, a'o
Rotation of joints, 1S7
Rouleaux, red corpuscles collect in. 62, 66
Round ligament. 1S6
Running, how efifected, 190

INDEX. . 397

Saccl'LCS (a little bag) hemisphericus, 219
Sacrum, os (the sacred bone, because offered in sacrifice), 11
Saline matters, coagulation retarded bj-, 69
excretion of, 3, 16, 107, 113
in food, 145
Saliva, action of. 153. 165

ners'ous centre for secretion of, 297
secretion of, 141, 152, 155
Salivary glands, 140
Salts of lime in bone. 10. 330, 338
Sarcolemma (ffoip^. flesh ; \€ixfj.a, a bark or skin). 351

absent in unstrii)ed muscular fibre, 354
Scala (a ladder) of the cochlea, 223
Scales of epidermis continually shed, 120, 309
Scapula, II
"Schwann, sheath of," 356

white substance of, 358
Sclerotic (cr(cAijpbs. hard), 253
Scurf, nature of, 309

Sebaceous (5£'^«;«. suet) glands, 120, 140, 314
Secondary- digestion, 131
Secreting cells of kidney, iiS
Secretion of tears, 264
Secretions entering the intestine, 164
by glands, 141
of the mouth, 152
Semicircular canals of ear, 217
Semilunar valves, 38
Sensations, 201 ei stipra, 278
auditor^', 235
compound, 266
simple. 265
subjective, 203, 268
Sense of hearing, 215
musoilar, 203
of sight, 241
of smell, 211
of taste. 209
of touch, 206
of warmth, 209
Sense-organs, 14, 204

essential and accessory parts of, 207
Sense-organules described, 205

connection of sensory fibres with, 252
of taste, 210
of touch, 207
Sensorium, auditor^-, 235

\nsual, 246
Sensorj' or afferent ner\-es, 201, 283

collected into the posterior roots, 360
indistinguishable from motor, 350
Septum (a partition ; sepio, I fence in) of the nose, 211
Serous ca\-ities, peculiar epithelium lining, 318

membranes, 34, note
Serum (whey, buttermilk), 34. 68, 71, 75
Sex, mechanism of respiration varies according to, 93

398 INDEX.

Sex, voice varies according to, 197
Shaft of bones acting as levers, 174

ossification of, 336
" Sheath of Schwann." 356
Sheep, heart of, examined, 32, 37, 39
Sighing, 94
Sight, long, near, and old, 260, 261

sensation of. 205
Single vision with two eyes, 275
Skeleton (o-»ceAAa), I am dried up), 10

weight of, 365
Skin, blood not rendered venous in the, 124
a double integument, 8, 309
an excretory' organ, 16. 369
kidneys affected by state of the, ii3
a source of loss to the blood, 119
weight of, 365
Skull, 6

formation of bones of. 339
number of bones of, 11
Smell, organ of, 211
" Sniffing," 94

air drawn into olfactorj' chamber by, 215
Sneezing, 94
Soda in bile, 129
Solids of the body, 366
Solidity, judgment of how formed, 276
Solubility of peptones, 159
Sounds, cardiac, 46
musical, 236
perception of, 216
respiratorA", 99
Specific gra\aty of blood, 72
Spectra, auditor^', 26S

ocular. 269
Speech, mechanism of, 197
Sphincter (<r(^iyyw, I throttle or bind) muscle of bladder, 112

of rectum, 164
Spinal accessor^' ner%'es, 291, 296
column described, 6, 279
cord, described, 279, 361

acts as independent nervous centre, 14, 288
effect of galvanism on, 14, 282
fissures of, 279
grej' matter of, 282, 289
result of injury to, 13
transmission of ners^ous impulses by, 28O
white matter of, 282
vaso-motor centres in, 290
ner\'es, 280, 290, 362
Spleen, 6

its office not understood, 134
Splenic artery and vein, 134
Spongy bones of nose, 214
Spot, blind, of eje, 247

germinal, of oNOim, 306
yellow, of eye, 243
Squinting, double vision a result of, 275

INDEX. • 399

Stapes (a stirrup), 229

its attachments, 234
Stapedius muscle, 231

possible use of, 239
Starch as food, 145

converted into sugar in alimentary' canal, 133,
by pancreatic juice, 165
by ptyalin, 153, 156
Starting at noise, a cerebral reflex action, 301
Stereoscope ((rrepebs, solid ; crKonem, 1 view), 276
Sterno-costal cartilages, 319

embrj'onic growth of, 322
Sternum {aTepvou. the breast), 87, 95, 179
Stiffening of muscle after death, 172
Stimulation of nerves, 141
Stomach {aTOfJia, a mouth), 156
Stratum comeum and mucosum of epidermis, 309
Striped muscular fibre, 171. 348

in heart, 36
Structure cancellated, of bone, 329
Sub-arachnoid space. 279
Sub-dural space, 279
Subjective sensations, 268, 269
Sublingual gland, 152
Submaxillary gland, 152

Suction pump, respiratorj' machinery regarded as, 97
Sugar in blood increased by injury to the medulla oblongata, 297
cons'ersion of glycogen into, 133
as food, 145

starch converted into, 153, 159, 165
Sulc! of brain, 294
Sulphur present in bile. 129

sometimes present in proteids, 144
Sulphuretted hydrogen, mode of action as poison, 104
Supination {sitpinus. lying on the back) of limbs, 184
Supplemental air, 94, 36S
Supra-renal bodies, 134
Swallowing, 155

ner\'ous centre for act of, 297
Sweat, iig

glands, 120, 314

stimulated by warmth, 137
" Sweet-bread." 6
Sylvius, fissures of. 294

Symmetry (<ruv, together ; ixerpov, a measure) bilateral, of body, 5
Sympathetic (avv, together ; TrdOos, feeling) nerve, blushing governed by, 53

system. 6, 278, 302
Synovia (ooii', with ; wbv, an egg), and synovial membrane, 11, 181
Syntonin ((TVt', together; retVw. I stretch), 144, 173
Systole ((Tvo-TeAAw, I draw together, contract), 42

Tactile {tango, I touch) corpuscles, 207

impressions, education of the eye by, 271
Taste, complexity of sense of, 211
organ of, 209

400 INDEX.

Taste-buds, 210

Taurocholic (ravpo?, a bull ; X'^^V' bile), acid, 129

Tears, secretion of, 264

Teeth, 22, 150, 341

development of, 344
enamel of. 154, 344
Temperature of blood, 17, 72

of body, due to oxidation, 17

regjulated by blood supply to skin, 54, 136
effect of, on coagulation of blood, 69

on vaso-motor nerves, 138
of expired air, 86

sense of, relative rather than absolute, 209
T&m'poTa.l (tempora, the temples), bones, 228
Tendo Achillis, 327
Tendons {tendo, I stretch), 188. 348
Tensor tympani (stretcher of the drum) muscle, 231, 239
Teres ligamentum (the round ligament), 182
Terror, its effect on the vaso-motor system, 53
Thaumatrope (6avixa, a wonder ; rpoTros, a turning), 274
Thoracic duct, 28
Thorax {Oupa^, the chest) described, 87

organs within the, 6
Thymus body, 134
Thyroid (Ovpeix;, a shield ; eT6of , shape) body, 134

cartilage, 191
Thyro-arj'tenoid muscle, 194
Tibia (a pipe or flute), 179

Tickling, paralysed limbs not insensible to, 14, 286
Tidal air, 94, 368

effect of change in, 102
Time required for propagation of nervous impulse, 285, 369
Tissue, connective, conversion of food into, 148
examination of, 323
adipose. 328
cartilaginous, 319
epithelial, 308
osseous, 328
muscular, 347
nervous, 355
Tissues, combinations of, 30S

minute structure of, 304
reproduction of, 19
various, 307
Tongue, 150

nerve supply to, 209, 297
speech possible after amputation of, 199
Tonsils, position of, 152, 210
Tooth sac, 34S _

Touch, retinal impressions corrected by, 272
sense of, 206

varying sensibility of different parts of the body to, 208
Trachea (arteria trachea ; rpaxv?, rough : the rough artery), 83

ciliated cells in the, 171
Transfusion, 75

Transudation through capillaries, 23, 25, 77
Trapezium (dim. of rpan-efa, a table), 182
Tricuspid (ires, three ; cuspis, point of a weapon) valve, 36

INDEX. 401

Trigeminal nerve, 291, 296

"Tripod of life," 20

Trunk of spinal nerve, effect of irritation on, 282

Tube, double, body considered as, 8

Eustachian, 152, 229, 240
Tuning fork, vibrations of, 237
Turbinal (turbo, I whirl) bones, 214
Tympanum (TVitxTrai/ov, a drum) of ear, 224, 228

IJlna (wAerrj. the elbow), articulation of, 184
Uuconifortalileness, sense of, 203
Unstriated muscular fibre. 171 _

in alimentary canal, 174
in bladdei', 112
in coat of arteries, 24
in fibres of iris, 174, 255
structure of, 354
Urea {ovpov, urine) excreted by kidneys, 3, 16, 107, 11:
secreted in tubules of kidney, 119
weight of, passed per diem, 369
Ureters, 112
Uric acid, 113
Urine, composition of, 112

secretion of. influenced by state of skin, 118
Utriculus (a small bag) of ear, 217

otoliths in, 221
Uvula (dim. of nva, a grape), 152 •

Vagus (wandering) or pneumogastric nerve, 55, 296
Valves in arteries, 27

course of circulation governed by, 57, 100
ileo-c£ecal, 161
of heart. 36 et sitpr-a
in lymphatics, 27, 29
in veins, 26
Valvulse conniventes, 163
Varnish, result of covering the skin with, 125
Varolii, pons, connection of, with cerebellum, 292

grey matter in, 294
Vascular system, 22 et supra
Vaso-motor centres in spinal cord, 291
nerves, 25. 53, 138, 290

ultimately traceable to spinal cord, 290
Vegetable diet, result of, 74
Vehib, 15 et supra

collapse when empty, 25
no pulse in, 47
valves in, 26
Veins, azygos, 34
cerebral, 26
coronary, 33, 39
hepatic, 31, 33, 127, 130

D D

4o2

INDEX.

Veins, Innominate, 28
intralobuL'ir, 127
jugular, 28, 35
portal. 26. 33, 130
pulmonarj', 26, 31, 34
splenic, 134
subclavian, 28
Veinlet, intralobular, 127

Velum (a curtain), the, 152 ,

Vena cava (the empty vein), Inferior, 31, 32
superior, 28, 31
Vena portae, 31, 33, 163

absorption of chyme Into, 161
office of, 51

peptones and sugar carried to liver by, 167
ramifications of, in liver, 126
Venous blood, dark colour of. 77

effect of, on brain, 103
Ventilation, necessity of. 105. 369

Ventricles {7'cntric7this, a little belly) of the brain, 292, 362
of the heart, 35

contraction of, 42
thickness of walls of, 44
Ventriloquism, effect of, due to suggestion, 270
Vermiform {vermis, a worm) appendix, 162
Vertebra; (jierto, I turn),_ bodies of, 6

coalescence of, in sacrum and coccyx, 1 1
of neck, 183
Vertebral column, as example of imperfegt joints, 180

foramina, 280
Vesicle, germinal, 306
Vestibule (or porch) of ear, 219
Vibrations, auditory hairs affected by, 221-231
In endolymph, 235

of ether, physical basis of light, 246, 251
molecular. 233

musical sounds due to regularity of, 236
of the ossicles, 233
sensory, organs affected by, 14
sonorous, 216. 221, 232
of tympanic membrane, 231, 234
of vocal chords, 196
Villi (j'illits, shaggy hair) prolongation of the la'cteals Into, 29, 163

absorption by means of, 166
Vision, conditions of, 251

explanation of the singleness of, 275
probable seat of end-organ of, 300
Visual sensorlum, 246
Vital actions, 2, 18

foods derived from the vegetable world, 169
ultimate analysis of^ 145
Vitreous {x'iirnm, glass), humour, 254
Vocal chords, 82, 191, 193

varying tension of, 195
voice due to the presence of, 190
Voice, production of, 190
quality of, 197
fange of, 196

INDEX. 463

Volition absent where brain is absent, 287

Voluntary- muscular contraction, brain the source of, 14

Vowel sounds, how formed, 198

Walking, mechanism of, 1S9
Walls of vessels, differing structure of. 24
Wandering cells, 326
Warmth, sense of. 208
Waste, in contraction of muscle, 149
nitrogenous, 146
as result of work, 3, 4, 15
Waste matter in blood, excretion of, 106
Waste products of work in tissues, not all useless, 107
Water, absorption of, by the large intestine, 167
excretion of. 3. 16, 107, 124. 148, 366

by kidneys, 112, 113, 117, 369
by lungs, 16, 87, 368
by skin, 119, 369
proportion of, in bile, 129
in blood, 72
Water-camera described, 253

ej-e-ball considered as, 257
Weight, proportional, of component parts of the body, 365
White matter of brain and medulla oblongata, 294

of spinal cord. 282
Winking, a cerebral reflex action, 301
Wisdom teeth, 347
Work, physiological, 2-5

estimate of, in foot-pounds, 366
waste a result of, 15

Yellow spot of ej-e, 243

width of cones In, 370
Youth, bones afterwards united, are separated in, 11
respirator^' process most active in, 102

Zona pellucida of ovum, 307

Zootrope {^^ov, an animal ; TpoVos, a turning), 274

THE END.

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